Shortly after the official announcement of the jumbo smartphones, a number of sources have reported that Samsung simply doesn't have plans to offer the 5.8" Mega in the UK. However, the gargantuan Mega 6.3 will be sold there. Still, if you really wanted the Mega 5.8 and nothing else, there's still chance that you'll be able to find it at some third-party retailer.
Samsung plans to launch the Galaxy Mega 6.3 in mid-May, so that's when we would expect the Mega 5.8 to make an appearance as well, although not through Samsung directly.
Meanwhile, if you want to get a better idea of just how big the Galaxy Megas are, check this post out!