i can tell you about german customs, i think it will be similar in britain:
you can bring 250g from outside the EU to your country within the EU without paying anything.
if you want to bring more, you can pay a flatrate fee of 40€/1kg but if its more than 5kg they will deal with it like its for trade, so you cant import it.
if you try to avoid the fees and they get you by passing the "green light" at the customs you will have to pay the complete taxes + a surcharge of the same worth. so they will cost more than buying it in your country