Actually it's not horrendously expensive in Harrods, well I don't think some of the stuff in there is any more expensive and in some cases it's cheaper than my local tack shop.
Harrods and Swaine Adeney Brigg 54 St James St are in central london. Decathalon stores sell v cheap jodhpurs and they are all over London. Lillywhites closed their (very good) equestrian department about 2 years ago. Depending on where you are there are others on the outskirts, like Frogpool Manor in Chislehurst, Kent which is about 3/4 of an hour by public transport from central london
You also find that many of the riding stables have tack shops (eg. Lee Valley)...
There is also a tack shop in Chingford on the high street (something like west essex... try www.yell.co.uk and search for riding equipment near your postcode?
What area of London are you looking for a tack shop in? I can probably have a think for more places!
wow! I'll save that for another day - £175 a pair! Maybe when I'm rich...
I work in Central London so as long as places are near a tube I can generally get to them. I might try and get to Decathlon at the weekend - some bargains there!