Buying a BRAND NEW car in the UK

If I buy a brand new car from a dealership in the UK, what does the price include? Do I still have to pay for license plates, taxes, etc? How much more am I going to have to pay to get the thing on the road?

As I am pretty rich, I will naturally be buying something expensive like a Mercedes E class.

What you look for is the "on the road price". The headline price for new cars rarely includes the licence plates nor does it include the delivery cost from the factory - which is a fixed fee even if the factory is literally next door to the dealership. Then they might charge for preparing the car - ie taking off the plastic covers, doing a basic safety check & buffing the paintwork and for putting a gallon of petrol in the car.

If you want to save a bit of money then look for "pre-registered" cars which are cars that have been registered by the dealership so they make sale targets but have not been driven on the road or had a "real" owner.

Suggest you start with a copy of What Car and look at the Ads in the back

Why buy a car when you can get chinese take aways delivered.

Nowadays, most advertised prices are 'on-the-road', including all compulsory charges. Road tax may be for 6 months or 12 months, and the amount of fuel they put in may be either full tank or enough to get you to the nearest service station. The only extra you have to budget for is insurance, which may be expensive and/or difficult to obtain if you don't have sufficient UK driving experience. For a car like Mercedes E-class, they are in higher insurance groups, around 15-17 (20 is highest for something like Rolls Royce, Ferrari, MB S-class, Porsche etc). You can usually negotiate a discount on list price, but only a tiny amount (maybe a few hundred pounds) for a newly-introduced and desirable model like E-class. As stated, nearly-new, pre-registered or 'demonstrator' car will save you more.
Look at What Car, Parkers, or Auto Express - also available online. Or AA Car Buying Guide.

I was also going to add that for expensive German 'executive' models like MB and BMW, cars in showroom already come with a range of optional extras, such as special colour/trim, sat nav, performance enhancement, uprated in-car entertainment and so on, so the sticker price will be considerably higher than a standard model. You can specify a standard model, but it normally has to be built on order, with a delay of several months. Also such cars may be more difficult to sell on, as secondhand buyer expects optional extras to come as standard.

"As I am pretty rich, I will naturally be buying something expensive like a Mercedes E class. "

so why are you bothered about the sundires then?