Hello Stars,
I live in London too so hopefully I can pass one some useful info! There are generally very few dedicated scrapbooking stores and certainly nothing like you have in the US. Cardmaking is a very popular pastime here. Scrapbooking is still relatively new here (but rapidly gaining on popularity), but we don't get large value packs of materials etc. like in the US.
A lot of small local arts & crafts stores now carry some basic materials such as punches, basic cardstock and paper, albums, stickers. There might be a shop like that near you but I have not really been to many local shops. (I sometimes go to the Wheatsheaf Art Shop in Baker Street.)
Outside London, I have been to the Craft Barn in Lingfield (which won the "Best Craft Shop in the South East") and Creative Pastimes near Limpsfield (which won the second best shop award in the South East). Both are quite small but have a reasonable range of (in my opinion) overpriced goods. Every now and again, there are sales in these shops, but nothing too spectacular. And certainly there are no American-style coupons...
There is also the QVC shopping channel which sells mostly sets of good quality materials. This Sunday, there is a 24-hour craft day, starting at midnight, with some scrapbooking slots - you can watch on freeview, cable or the internet. It is a must watch for me :-)
Personally, I tend to get my scrapbooking supplies from online shops as they tend to be cheaper and there is more and more of them and they often have newer stuff. Also service is generally very good, I mostly get my goods within 2-3 days.
I hope you can find what you are looking for & good luck!
Klara
P.S. I am a 32-year old Czech/South African living in London... (new to scrapbooking but addicted) If you would like to get together and talk scrapbooking, feel free to contact me on klarajw AT tesco DOT net