I suppose this can be dependant on the type of fish you're gonna be keeping, but I think most fishes won't mind either way.
Another question you may need to consider it how easy would sand/gravel be easier to clean. Personally, I prefer medium-sized gravel. Better aeration and cleanability.
Look for pool filter sand. These aren't really sand - rather, they are small grains perfect for the aquariums. Its small snough not to trap much crap and large enough that they don't get sucked into the vac easily.
Tried sand, tried gravels of different sizes. PFS is the easiest to maintain and also looks very good. (See the picture of my 25g tank).
Hmm... I did live in UK for about 7 years of my life, and if my memory serves right, it isn't the best place to have outdoor pools... Perhaps you can call some DYI shops to see if they carry them? There must be some stores who carry some pool supplies for those rich nabours who can afford indoor pools...
Yeah I guess logically there must be _some_ pools in the UK and they must get their stuff from somewhere, otherwise they couldn't exist....I'll have a look. Thanks for your help.
Also, at least in Canada, this stuff is much cheaper than the aquarium gravels. I purchased a bag for $5CA, and I used up less than half of it to cover 25g tank. It is only slightly more expensive than sand.