yes playsand from argos is a good choice, try not to go much over 2" depth, this will stop anerobic bateria growing and causing futer problems. you need to wash the play sand quite a lot, this is what happens if you don't (click thumbnail for lager view)
If pool filter sand is in your budget, I'd go with that as it requires very little, if any, washing.
for pure ammonia in the UK, I've heard of people getting it from Boots superstores (not small high street ones) or Robert Dyas. You need to make sure it is 100% pure, with no other addatives, look in the cleaning products aisle.
guppies and neons is a good choice, and in that sized tank you could have 6-8 of each.
An angelfishs' natural food are neons, so it might not be a good idea to keep both together, although some people have with no problems, the risk is yours to take.
Neons are also a sensative fish, so wait untill the tank is matured (around 8 weeks after cycle) before adding them.
Good luck and lets see plenty of pics