I think AE are the only UK supplier for ADA stuff, so you may have trouble getting another source.
As for tropica, the only other place is a Maidenhead Aquatics on there planted section, Apparently one of their stores has gone 'Planted' and is offering tropica plants and ferts/substrates.
It's really worth going to AE for your stuff, as Richard genuinley cares that his customers are happy. This is from personal ecperirnce and from others here on the board.
HTH!