if i needed a load of resin / filler i'd go to a directly to a paint suppliers (you should be able to find some after a search through yellow pages). halfords arent that bad in all honesty, but if you need more than 2l of both your going to make a good saving by avoiding them. if your friends with anyone who runs a bodysho get their supplier information - thats how i found out about a mint cheap as chips supplier in stoke.
if its paint auto-paint.co.uk are really good. not particularly cheap, but ive never had a problem with any of the paint they've supplied. they also do all the full size tins if your using 2 pack paint etc.