The reduced flow rate is better than none

Is the media clogged? Try cleaning it to increase flow

If that doesn't work, the pump/filter needs replacing, as the pump is on it's way out and may only be a few days away from complete failure
Eheim are rated the best exturnal on here, with the Pro2 range being the best IMO. The classics are proven, but less flexible... The pro1 range has like no features, see it too ways; less to go wrong or just harder to work on? Pro 3's haven't been round long and are as yet un-proven... Eccos are unrealiable IME and in the expreience of other members, and thermo filters from any brand are un-reliable....
On £100 budget, getting a suited eheim may be tight..... If you find that £100 doesn't streach to a suitable pro2 model, I'd get a Tetratec EX700, that can be picked up form £80 in-store or £60 delivered on-line, well within you budget
Fluval are proven to be un-reliable, with like 50% of users reporting catestrophic failures of the main seal ring, or aqua-stop seals just outside warrenty, which drain their tanks onto the floor as a result

IME they also chanel, and they clogg within 2 weeks in secnarios where every other filter will go 6 months without a clean..... I hate them with avengance after my 404 electric shocked me, and had the main seal ring fail for the second time in 6 months......Take your pick
When you get your new filter, dump all the old filter media into a basket, and then pack arround it with new filter material, to instantly cycle the new filter and ensure ammonia doesn't spike after the old Juwel internal finaly packs up
HTH
Rabbut