Well if I were in your shoes having done a bit of research you might like to Google seaton submersive, rythmik (sp.) Subwoofers of Hsu subwoofers. All based in the states but export to the UK and have UK voltage versions.
Import duty on a powered sub is 4.5%. VAT at 20% and customs clearance charges also apply. Then you have shipping. All US retail prices are quoted excluding VAT.