What about Colorama? they are just around the corner, Silex Street I think....it's the actual processing place....opposite the borough poly there is a turning thats where it is..I'm sure it's Silex Street, you can take them into the counter...
Can't remember the name of the place, but there's a photographic shop by London Bridge station, on that raised walkway above Tooley Street where Richer Sounds is, and they do 120 developing; I had some 120 trannies done there, and they were fine, though can't can't remember the cost.
There's also a little place on the corner of Weston Street and Snowsfields, next to the bookies, never used them though, so no idea as to cost, quality etc.
Otherwise do the unthinkable and walk across the river to Jessops on Cannon Street, or hop on the Northern line to the Jessops at Moorgate, its not that far honest! They are very cheap for 120 developing, but don't expect great quality, and I think it takes about a week or 10 days.
Metro is a pro-quality place and are over the river in Clerkenwell...usually very high quality and open 24 hours. Not that cheap but they are reliable and very fast.
76-78, Clerkenwell Road, London, EC1M 5TN
Tel: 020 7865 0000