As PHAEDRA has noted, much of the 'US' powder is made in Australia, including my beloved XBR (8208). It is shipped only once or twice per year from US to GB, dangerous goods are lumped together for the journey for licensing reasons. There is nothing much you can do about it since the UK is an extremely small market for firearms, ammunition and ammunition components. To put it in perspective there are single outlet (non-franchise) shops in Arizona and Queensland who sell more Berger and Sierra bullets and powder brands than the entire UK in a year.
Beware cheap prices from distance sellers as invariably SOME retailers are not letting the courier companies know it is dangerous goods cargo. A good way to catch the eye of legislators once again for the wrong reasons - self police and although its hard not to be cost driven, we all know roughly how much we should be paying.
I would definitely urge handloaders to try RS powders. I do know the distributor well, and he ALWAYS has stock in ALL lines having never run out. If your 'local dealer' says otherwise he is fibbing. Many have just given up stocking.