I think quite a few UK retailers sold ADA products at one time...I also think the reason they don't could possibly have been more to do with the distributor's business strategy rather than anything else.
I could well be wrong, but I suspect the strategy was to control the supply chain and monopolise the market, probably by inflating prices to the point where retailers had little or no margin on the retail value of the products. And/or ADA set the price point too high in the first place. In which case James may have decided it'd be more efficient to use the business as a vehicle to promote his aquascaping career.
Either way the result is possibly the same - availability becomes limited, and in an emerging niche market there is always the risk that this will in turn throttle your own market place, especially when customer service isn't always on point, and when there are a growing number of similar products continually entering the market at a more competitive price point...and perhaps more importantly, are widely available.
If anyone is going to make a go of distributing ADA in the UK, I guess they'd need to get it out to as many retailers as possible and at the right price point. I suspect this will prove incredibly difficult, because the average UK hobbyist shopping in their LFS, or even in the Maidenhead Aquatics chain, isn't necessarily going to appreciate the wonders of optiwhite glass and the finer points of seamless silicone work, for instance. I get the feeling it would require an enormous amount of education and marketing...which equals an enormous investment in time and money.
It would be considerably easier if the price point of ADA products in this country matched those in Japan. I doubt ADA would consider that, for one thing they'd be worried about devaluing the brand. But if they did, any distributor would still probably have to import a vast amount to gain the economy of scale necessary to make it worthwhile, and that'd be a huge risk. Not least having a huge amount of capital locked up in product that potentially no one wants.