Interestingly, their music is being released in the UK (where I live). They've even had a gig in London. I doubt they'll have much of an impact here as they probably had in Japan, because.. 1 - They're Japanese. Sure PSY was popular here but Gangnam Style was a fad. It's not very common that we get stuff from Japan and when we do it's not guaranteed to be big here. Anime is only released directly on DVD to a niche audience here, it's not on any mainstream TV channels (imagine watching One Piece on ITV?
), and Tokusatsu we only have in the form of Power Rangers. 2 - Heavy Metal doesn't seem to me as big in the UK as it used to be.
I really like them, I first found out about them when searching nyaa for music it was round the time Ijime, Dame, Zettai came out I remember downloading it and thinking it was ok and left it for a couple of months and then listening to it again and getting hooked.
I tried to get to their concerts when they were over, they had 2 solo concerts (both sold out in minutes) and they played at sonisphere so it doesn't surprise me that they are selling well.
There has also been a ton of anime on itv and the BBC: Pokemon, monster rancher, digimon, cardcaptors, Shinzo and yugioh to name a few, granted the mainstream show very little now but the kids channel are constantly showing dbz, yugioh etc. there was even a time were BBC 2 showed a bunch of studio ghibli movies in a row on multiple days. Hell I still remember back in the early 2000s channel 4 having a japan/animation week and they should a few Godzilla movies, an Adam and joe animation special and most importantly for me at least the shadow skill movie.
The problem isn't that it's a niche market the problem is that shops like hmv etc only ever buy in the same dvds as everyone else so you end up with an anime section which has about 30 copies naurto, bleach etc and one maybe one or two titles of something else and that's it.
As for heavy metal not being as big anymore I think it depends on what part of the country you are in. Just before I moved out to New Zealand I was living in Winchester and there were quite a few heavy metal concerts happening in and around the general Hampshire area and I even when and saw I am I's debut concert in Birmingham. There were also a bunch of festivals such as sonisphere and download all getting massive crowds going to them, you might want to check those out while you are at it if you are looking for some more British heavy metal bands to listen to you might want to check out I am I, primitai and weylander.
