You'll have to pay anything from 4% to 17% import duty. You need to check at
Duty.
Click on Browse and then find what you're looking for. I went Section XII, Chapter 64 but then it depneds on what the trainers actually are. I went 6402 then 12 then 19 and get 17% after picking USA from the pulldown menu.
....then if the value is more than £18 (from a website) or £36 (if a gift from one private individual to another) 17.5% VAT....
and then £4 for the P.O. to get them through customs. But only if there is duty to pay. There's duty on DVDs but if they declare the value is less than £18 then it usually comes straight through no problem.
So your £30 trainers end up costing £45
The sender has to fill in a form that states what the goods are and their value. If they lie then you are responsible and can be fined and/or have the goods confiscated.