Cheapest way to get from Soho to Gatwick airport by 05:30am

In general, the cheapest way to get to Gatwick is by bus, not train. In the case of Heathrow the tube is mostly going to be the cheapest, but a bus shouldn't cost much different. So for Gatwick it reduces to finding out whether buses run at this time.

http://www.gatwickairport.com/to-and-from/by-coach-or-bus/

I leave you to research the bus timetables of the various operators, and where in London they arrive at.

Is Gatwick express my best bet?

Thameslink trains also run at night to Gatwick. So you could take a train from either Blackfriars or St Pancras (the former is a less scary station), and it would also be cheaper than Gatwick Express. But probably not cheaper than the bus, though it is always difficult to be definitive about these things given how cheap fares for advance booking vary so much. However check the precise date, there can be engineering works preventing this journey, for example at the weekend. Gatwick express can also be closed due to engineering works some days, so use nationalrail.co.uk for your precise date of travel, and check again nearer the time.

Cheapest method to get to Victoria station from Soho

Walk is cheap and practical if your bag is not so heavy and you really are trying to save every last pound, though probably takes best part of an hour. Otherwise cheapest is night buses. (Note that you can't pay by cash for a local London bus, you have to get an Oyster Card. This is also the best way to pay for the tube to/from Heathrow. Research tourist Oyster cards on the tfl website.) Go to a bus stop and you will probably see the night bus network maps etc, or check online. But on Friday and Saturday nights, some underground trains run all night, and that may be more convenient/reliable given precise arrival times may be involved.

3) Does it make more economical sense to: a)go from Gatwick to Soho, sleep a few hours (25 quid), then Soho to Heathrow? OR b) go straight from Gatwick to Heathrow and sleep at a Yostel (70 quid) in the airport

Option a should be about £20 cheaper, at least, if you are careful over your routing. But will involve you in a lot of arranging.

I am seeing conflicting fare prices from Soho to Heathrow (seeing as low as 6 quid to 35 quid. Very confused on this)

£6 is probably the tube. Other things can cost all sorts of prices depending upon routing, when you book, etc.

Take the train from Gatwick to King's Cross, here you will find both cheap hotels and hostels. Then take the tube from King's Cross to Heathrow. These are two cheapest transport options, and King's Cross is pretty central just a short tube ride into the west end or a not too far walk. The benefit of a hotel is after check out you can leave your bag until setting off for Heathrow, not sure if a hostel allows this.

There are direct flights from London Luton to Prague and Brno in the Czech Republic and Bratislava, Kosice and Poprad in Slovakia. London Stansted to Prague, Brno and Ostrava in CR plus Bratislava and Kosice. London Gatwick to Prague and London Heathrow to Prague. So plenty of choices for flying into one country and out of the other. Flying from Luton or Gatwick and back into the other airport isn't much of a hassle due to the train link between them but if flying from Stansted or Heathrow it's best to return to that airport.

If you hire a car in city A and return it in city B, you will almost always pay a premium. It is cheaper to return the car to the same place you picked it up.

+/- one hour doesn't work either because the dude covers lots of clicks in that time - but where exactly he is in those 'two hours' can vary a lot depending on his deliveries.
The majority of their customers are businesses, where deliveries can be done anytime during working hours.

If you're at Heathrow airport give the service point as address and pick it up there.

Frankfurt, Leipzig, Gatwick, don't worry about how they make money on that; they use a 'hub' system and even if it seems to you as not making any sense I'm sure some experts calculated such 'hub systems' to be more efficient over the entire volume.

Worked for Swiss Post as well, same hub system, a letter from your own village goes into the town hub, then into the big hub - and then back the same way...
Unbelievable, stupid you might say but I'm sure they did put some serious calculating in there and financially it makes sense over the entire logistics volume...

Does anyone know how much time in between my flight landing at Gatwick and taking the National Express bus to Heathrow I should give? I land at 23:40 and the only sensible option it is giving is 00:35 which seems like it's cutting it slightly close. The next bus after takes three hours and the next bus after that is 02:40. Is 55 minutes ample time, assuming my plane is on time or even twenty minutes late?

You can check out Gatwick's website yourself for options:

http://www.gatwickairport.com/to-and-from/

To get to Paddington, you'll have to take a train or coach somewhere else, and then transfer to the tube. The tube's the easiest way to get to Heathrow. Paddington isn't the most convenient, but why not? All in all, Victoria's probably the most convenient area, but as long as you don't have a problem with a tube ride, it doesn't really make a big difference. Anywhere in Central London will be fairly easy to access both airports.

Narita is far from the best,but you were still unlucky. ..or lucky at Heathrow.

Yes,all the London airports are bad for queues. ...Gatwick can be even worse than Heathrow on occasion. Depends a lot exactly where you arrive, and at what time.....last summer I think we were in line for passport check about 30 minutes, which is reasonable by London airport standards. ...

There is NO train from Heathrow to Gatwick and the one that goes from Victoria is for sure not free.
I can't understand how you got that idea from reading Iviehoff's post.

Added - Ah! I see.When he said that "you can transfer between terminals without charge, with a bicycle, boxed or unboxed, using the mainline trains - Heathrow Express and Connect" he was talking about HEATHROW terminals - not terminals at different airports.
You can't transfer between the airports - free or paid - by train.

"can't jump back and forth to each receivers liking"
Not asking them to do that, just tell 2 time + or - 1 hour when then they will be delivering. Other companies in the UK give a time and some even a link that shows on a map where the driver is. I think the sender has specified a business delivery, it would seem that there is more leeway had it been a person delivery. They stood fast on not redelivery however since getting back to them and explaining this is a personal delivery they say thay can do it.
The ironic thing is I am working at Heathrow and the package has already been routed through here once, as well as Frankfurt, Leipzig, Luton and Gatwick. I can't see how they are making any money on this.
FCG and Norrie19. As the sender had specified the delivery address and had specified no delivery to any other address DHL would not budge but they seemed to have relaxed that stance now. I will point out to the sender that next time they send items as a personal item not a business item.

wouldn't uber be cheaper than a taxi?

Not sure Uber has scrapped its flat fares and added surcharges for Heathrow and Gatwick to cover parking fees.