If terrahash will declare full cost of devices which was sent to you(i assume they will)
 VAT will be applied to original amount of invoice (not sure how much it´s in U.K., but in Spain it´s 21%)
also some custom processing fees will be applied too.

I will recommend you switch to hosting instead, this is what i done, because custom can take up to one week(here in Spain it´s like this, not sure how it works in U.K. but i guess not really faster)

My personal decision was made on idea that i will exclude extra charges from customs, plus i can start mine earler on hosting so this will give some extra income, as time in this case have big influence, to much hash power will be released during august-september.