Not 100% but its about £35 and above, but dont quote me, I buy parts from the US and sometime I get caught for import duty and vat, (Boots struts for my probe with shipping £34 NO duty or tax, but then last order was over £100 had the pay something like £22 before they would deliver them once my item arrive in the uk..
I`ve asked the seller Rockauto`s to put it down as Secondhand and a value of $5 but they wont,
I also buy from Australia and tell them to just put a low value on the item (because costs to much to send items back) and I`ve never paid Duty or tax
You should find the information in the Gov.uk site