What's your definition of 'warm enough'? Barcelona should be very pleasant (with maximum daytime temperatures likely to be around 17C and probably a good deal of sunshine). It's easy to get to, with flights to Barcelona, Girona and Reus from nearly all main UK airports. It's got fantastic shopping activities, great bars and restaurants, tons of history and culture and a wonderful, cheap, fully-integrated transport system. It's also got some lovely beaches but it simply won't be sunbathing weather.
If 'warm enough' actually means 'sunbathing weather' then, to have any chance of that you'll need to go further south, with the obvious choice being the Canaries, where you're likely to encounter maximum daytime temperatures of around 21 or 22C.
To help us make further suggestions, please tell us just how warm you'd like it to be. It might also help if you can indicate which airports are convenient for you to fly from, as there's little point in us suggesting places that you can't easily get to!