Jonny, the amount of tax etc when importing into the UK goes like this:
VAT 17.5%
Import Duty 3%
............. = 20.5%
In addition you'll need to factor in shipping, which can vary greatly between $100 - $200.
Also you may have to pay broker fees of about £10, for storing the item.
So, take a guitar costing $1,000 that you have seen in the US.
The total you could end up paying can be broken down like this:
$1000 - VAT 17.5%
= $1,175
$1,175 - Customs Duty 3%
= $1210.25
Shipping - $150 (for example)
= $1360.25
Broker fee - $20
Total - $1,380.25
Your guitar has went from $1,000 to $1,380, which although quite a bit more may still be worth doing.
Hope that helps.