I can't think of any decent sub at that price and when testing the Mordaunt Short sub in the current edition, What Hifi said that it was the first sub under £250 that they had felt happy with but at £220 that is out of your price range.
If you are willing to buy second hand, keep an eye on the Classified forum here as some reasonable subs do come up for that price.
The short answer if you are talking new is No.You would be better looking at second hand, there are usually plenty of subs available both here and on e-bay that can be had for around this sort of money.
Look for something like Paradigm PDR 8 or 10 or better still a PS1000, other makes to consider if they come up around £100 would be REL,B&W, Mission, pretty much any model from these manufacturers will be miles better than a cheap £70-100 boombox that you would get new.
You are better off not having a sub, I made the mistake of going for a cheap Eltax Sub-10 subwoofer, great power, but not control, and it just boomed at everything!!
The Kef is so much nicer!!
No sub is better than a crap sub.
You would probably be better going for a speaker package, Yamaha NSP100's perhaps? They can be had for a bargain, and should sound better than the speakers you have now, but they are purely movie focused jobbies.
And the sub you get with it will probably be better than the celestion you have linked to above.
My Eltax is sitting around doing nothing now, I might use it as a second sub or might just use it for a surround system upstairs in the bedroom. If I could turn back the clock I probably wouldnt have bothered with the Eltax and saved for something better.