Hmmmm, I'm not sure I can totally be of help to you, as it's been several months since I was last in London.
Aperture's prices are some of the very best--I bought from them last summer when I was in the city.
Two other places I frequently bought film were out on the Whitechapel Road--one is a small shop just a couple of doors east of the Whitechapel Art Gallery, on the same side of the street (before you get to Brick Lane). I *don't* see film listed on their website currently (
www.totalblankmedia.com), but sometimes the website doesn't reflect what they have in the shop. And if you go just a few dozen metres east of Brick Lane, again on the same side of the road, there's another shop. I don't recall the name of it, but they had film prominently displayed in the window. I picked up a few rolls of HP5+ from them, and they had a rather large variety of film stocks. If I recall correctly, on that film they were very slightly cheaper than Aperture.
Another place, whilst your on that side of town, is
Teamwork Photo--their shop is definitely worth a browse in general for a photographer, and they carry lots of film. The prices are on their website, so you can compare them to Aperture and others.
Finally, my other occasional go-to spot--if you're in the Farringdon/Clerkenwell area--is
Process Supplies. Again, they have film listed on their website, so you can check prices. That's where I get all my film processing supplies--the name kind of gives it away, innit--but it's not as fun to just browse as Aperture and Teamwork as everything is behind the counter and you have to ask for what you want.
Really though, if you're right in Central, Aperture is probably your best bet. They (and Teamwork and Process Supplies) get extra marks from me, as they are entirely dedicated to serving serious photographers, and keeping film photography going. I've bought quite a bit of stuff from Aperture over a long time patronising them, particularly film, and they're good people in addition to having very competitive prices on film, and most things in general.