Is it that easy? or am I missing something ?
Unless you're registered for VAT, I think you'll find it doesn't seem to work like that.
Basically in the UK the seller will have to charge you VAT if you are not VAT registered. So you'll have 17.5% on the UK price. On top of this because it's a car that's less than six months the Revenue here will charge you the VRT (as expected) and the VAT @ 21% on the price again!!
I know you've already paid VAT in an EU state and legally shouldn't have to pay it again. However, this legal requirement seems to bypass the Revenue here. Either way you'll have to pay the 21% VAT here if it's new. Dealers who import don't have to worry about this as they are VAT registered.
(BTW registering for VAT is not a trivial matter and I wouldn't suggest you do it just for this transaction).
<edit> From the Revenue's website - Document on VRT
13. When is VAT chargeable and payable?
In the case of a new means of transport i.e. land vehicles (excluding agricultural tractors) that were supplied six months or less after the date of first entry into service or have travelled 6,000 Kilometres or less, VAT is chargeable and normally payable at the time of registration in the State, even where there is evidence, e.g. an invoice, that VAT was paid in the country of purchase.
VAT is also chargeable and normally payable at the time of registration in the State of vehicles (both new and used) that are imported from outside the fiscal territory of the EU.