Open all the interest paying current accounts, and all the decent regular savers.
That will deal with a not unreasonable chunk of it. For the rest other people can advise better than me. You could get approx 1% on instant access savings (West Brom do a 1.05% instant access), or around the same on premium bonds. You could take some risk like P2P/investments for higher returns (with potential for some loss). You could put Â£1000 into ratesetter for the Â£100 welcome bonus for example
It's really not worth doing this if you might need the money within a year. Just put it all in NS&I or split between three highest paying instant access accounts.