Travelling from London to Edinburgh/Edinburgh to Amsterdam

EasyJet is your best bet. There are other airlines that offer the same service - but EasyJet is quickest, fastest and theoretically cheaper.
Do be aware any hold baggage is charged. Check out their website.
As already mentioned - to get a cheap train, book on-line and get you e-ticket or machine/booking office pre-paid ticket before you travel.
B.A. do flights from Glasgow-Amsterdam taking just under 4 hours, at a cost of between £60 and £80 one way. Yes, a bit dearer and longer, but there'll be no hold luggage charge and you'll possibly get free food and drink on board.
It seems the Rosyth-Zeebrugge Ferry is still running on DFDS's website - but it also seems it'a impossible to buy a ticket....

http://www.norfolkline.com/EN/Ferry_routes/TravelUpdate_RZ/

BA Glasgow to Amsterdam requires a change in London. Not worth risking missing your connection. Could do KLM Edinburgh to Amsterdam 1.5 hours. Web site shows cost as £400+ single but book a return and forget to come back and you can do it for £78. I don't suppose you can claim back the return portion of the fare (£3) but legally you can claim for the unused service charges etc.
Yes the Rosyth Zebrugge ferry still runs. I see it when I walk my dog! - but it is as I said above - freight only.

I did mention that the BA flight takes 4 hours.
Thanks for clearing up the very confusing Rosyth Ferry website info.

I was at a trade show in Amsterdam two years ago but had to do a gig in Glasgow on the Tuesday night.
This is how it went:
Amsterdam Tuesday 4pm: Left trade show, jumped on train to Schiphol with handluggage only
Amsterdam Tuesday 5.30pm: easyjet to Edinburgh
Edinburgh airport Tuesday 6pm: pick up car which I'd parked there on Sunday night before flying to AMS
Drive to Glasgow, park and get onstage just as band is sound checking
Do gig, drive to Glasgow airport, stay at Travelodge, leave car there (for free)
Wednesday 5am: check in on KLM to AMS

It all went great up until then, the plane got diverted to Brussels to refuel because of unexpected weather, but I still was back on the stand at the trade show by 12 noon

Friday, flew back to Glasgow, picked up car and drove home.

That KLM flight is never cheap though. Easyjet's the way to go.

Selecting August 31st, Easyjet single flight paying by debit card with 1 piece of checked in luggage is £50. KLM similarly is £78 (return) so the statement 'Easyjet's the way to go' does not always hold true.. Another thing to take into consideration is departure times. KLM provides greater choice..

*15.... Where are you finding this £78 return with KLM?
The cheapest I can find is over £400 single.

www.klm.com - go for the return option rather than a single. I used outward 31/8 back 7/9 as a test. Don't blame me for the ticketing price logic - I'm only a prudent traveller !

thanks for all the comments!
i'm thinking now that we'll fly to amsterdam from edinburgh, just coz it seems a bit simpler.
still undecided about getting from london to edinburgh - i like the idea of train travel, i just wish i could book it sooner. can anyone give me an idea of an average ticket price? flying out of london seems like a hassle coz the airports are so far out.

also while we're on the topic, can anyone suggest good hostels in london/edinburgh/amsterdam? or even just good areas to stay/areas to avoid.

You'll struggle to find any advance train tickets over 8 weeks ahead of your travel time.
You can however, get a single from London to Edinburgh for around £12-15 on selected dates up to mid-May right now. So that'll give you a good idea of what to expect in August, or thereabouts.
I'll leave others to recommend a room in London and Edinburgh, but as for Amsterdam - I often stay here - and it is quite superb....
http://www.oldquarter.com/

Its a little pricey, but everything about it is just perfect.
For something quite a bit cheaper, I've stayed here too. It's less than a minute's walk from the one above....

http://www.hostel-meetingpoint.nl/index2.htm

A brilliant hostel and very friendly. Just beware though, they charge extra for plastic payment if you just turn up and ask for a room on the day. Get some cash out of an ATM first.