go for UK all the way!!!
L2 its best BB5 ever, strong, fast, hard punch good range. it has no backdraws against other BBs and u can armorwhore it.
the only "bad thing" about UK are SS... they can an will hunt u down first to every other nation just because of UK's shitty Bulge.
UK is for non-skilled players, any average player (like myself at BB lol) can make 100 to 150k every battle with a L2
RM its a great nation, fast ships good range and lazor beam angle cannon, but there will be a nerf, so nobody really knows what's gonna be after that...
For any other Nation:
US: fun BB's but BB4 and BB5 have low range (compared with same tier) and high angle, on the other hand they have a great punch. Best (along with UK's) BB6 around.
IJN: not so fun BB, they have a great punch, at the cost of low range, fat ass and glass jaw.
KM: good range, good AA and nowadays decent punch (at BB5) but really need skills as they are not so fast and quite fragile.
MN: shitty range, but great speed and hardest punch ever. fun for play as they are all-or-nothing playstyle BB's
SN: can't really argue, haven't tested