Aquatic Habitat in Gloucestershire stock quite alot of planted tank stuff, including pressurised CO2 kits, substrates and tropica plants. I'm sure they've heard of ADA (but I've never asked them!). They don't stock any of it though.
The problem with the average LFS though is that most of the people who go in there aren't at the 'serious' end of the hobby. So if they did stock ADA substrates, it would still be sitting on the shelf under a layer of dust 2 years later.