Is there a way to work out total fees for buying toys from the US?

[perceptionreality]

If it was $300 worth?

When I've bought things from overseas in the past the duty has equated to the VAT ie. 20%

Ah ok, thanks Cogito.

On theory you pay customs duty, too. Check HRMC website for payable rates. The VAT is applied on the total with duty. Suppose you pay $300 (call that £188). 10% duty item is now £207. Then you pay 20% VAT on that, or £228 in total.

Real duty may not be 10%, have to verify.