because I think I the UK you don't pay import tax on goods under £10-15,
No it's not import duty it is VAT and sometimes it is waved although strictly you have to pay it. There is also an £8.00 VAT collection fee that goes to the post office so often this is greater than the 20% VAT. Incidentally you also pay 20% VAT on the post offices collection fee.
Import duty is totally separate and is applied at the rate according to the category of goods. So sometimes this can tip things over the limit they decide to wave the VAT.
So for small purchases it can be good but don't be tempted to buy too much.