Simply put, you will be charged 20% vat on price of item plus shipping and import tax. To cover yourself add 30% on top of all items bought abroad. Basically, change Dollar to sign to GBP and depending on exchange adjust slightly!!
I've bought over a hundred watches from all over the world and rarely paid tax. Maybe some 10% GST claimed by eBay, but that was it, and not all the time.
ATM it seems that every side of the sale wants a bite.