Having done some research over the last few months, here's a rough idea of how I believe it works.
Customs duty is worked out on - doll price + shipping costs.
Import tax is worked out on - doll price + shipping costs + customs duty.
Customs duty can be from 0% - 85% but is usually between 5% and 9%.
Import tax is the same as VAT at 20%.
So....using Xe.com for currency conversion and for this example 7% customs duty, here's a rough total.
I'll use the Alicia 1H example
http://sinthetics.com/gallery/alicia/alicia-1h/ and the top 'buy this version' which is $5700.00 plus shipping $800
doll + shipping costs
£3,632 + £510 = £4,142
now add the customs duty at 7%, (doll + shipping) = £290
£4,142 + £290 = £4,432
now add import tax 20%, (doll + shipping + customs duty) = £887
£4,432 + £887 = £5,319
So the total cost would be £5,319
One other thing about customs duty and import tax, and I'll show you by extreme example.
You purchase your doll in January $5,000 (£2,500) while the exchange rate is £1 = $2.
She arrives in the UK in March and the exchange rate is now £1 = $1, you'd pay the customs and tax at the March rate.
So instead of paying x% of £2,500 as it was in January, you pay x% of £5,000 as it is in March.