Andi,
SWMBO & I take part in the H4H 4x4 European Rally each year as
Team Skibum. Our most efficient method of fundraising has been to stand at supermarkets, DIY stores, shopping malls etc rattling cans. All the big supermarkets have Community Colleagues (ASDA Title) or some such and are happy to host this kind of thing.
H4H give loads of back up with cans, banners and the rally provides public liability cover that tends to keep them happy as well. I'd suggest you get in touch with MacMillan and see what they can do to support you then start calling round as many stores as you can. You'd be amazed. I spent three days recently can rattling at Asda and we raised best part of