Lithril,
in UK biowheels are basically unobtainable - the marine wet-dry filters aren't ideal for freshwater tanks (at least according to my research) and are extraordinarily expensive. Until we can get Penguin Bio-wheels or other more reasonably priced brands in UK we're basically stuck with Eheim or Fluval, internal canister or external filter.
Of those I'd say the Fluval is the better internal canister filter, but the Eheim is by far the superior external - and externals are generally superior to internal canisters anyway.
I have seen a Dutch site that sells to UK selling an Eheim "hang-on-the-tank" filter but it has built-in cartridges that unavoidably contain carbon (not so good for your plants). It was quite cheap, however, even taking the Euro exchange rate into consideration. Click here for the
Eheim Liberty. Note it only mentions "chemical and mechanical filtration" so presumably it isn't a great bio-filter. Could be a useful addition to an UGF system, I suppose.
I'd suggest if you have £45 buy an Eheim Ecco 2231 external filter. I've never heard a bad word about them, except for the price. They are rarely sold on Ebay in anything like working order - unless they are new from dealers (who sell them at an average price more than you can buy elsewhere online), which suggests to me that the only people to get rid of them are those leaving fish-keeping altogether. Fluval externals, OTOH, are always being sold 2nd hand, usually because the owner has "upgraded". I'll leave it to your imagination what that means!