The UK HMRC charges no customs duty or import duty on imports of computers, but does levy 20% VAT (sales tax).
Using today's exchange rate of 1.6149, here are the UK like-for-like premiums for several Apple products in the UK:
7.6% - ipad 2
13.4% - ipod touch
14.4% - mac air 11 64gb
12.1% - mac air 11 128gb
13.9% - mac air 13 128gb
13.5% - mac air 13 256gb
12.1% - mbp 13
21.1% - thunderbolt display
Note: these premiums compare the US price excluding sales tax to the UK price excluding the UK 20% VAT sales tax.
Therefore, the UK premium is much larger when you consider that in some US states there is no sales tax, but the entire UK is subject to 20% sales tax.
Also note that VAT is only a sales tax on items for personal use. Business users do not have to pay the VAT (they reclaim it).
Here therefore are the premiums comparing US states with zero sales tax to the UK:
29.1% - ipad 2
36.1% - ipod touch
37.2% - mac air 11 64gb
34.6% - mac air 11 128gb
36.6% - mac air 13 128gb
36.2% - mac air 13 256gb
34.6% - mbp 13
45.3% - thunderbolt display
Lesson: go on holiday to the US, buy your Mac in a state without sales tax, and you're still ahead when you pay 20% tax on the import when you arrive back in the UK.