All of Radio 2's DJs are self-employed freelancers, so they're at liberty to negotiate whatever type of contract suits them best. (However it also means that they can be removed with little or no notice. c.f. Sarah Kennedy). It seems that SW has simply got a contract that pays him well enough to be able to take long holidays, as (for example) Terry Wogan now has.
Unfortunately he's due back next Monday (with Sara Cox sitting in for him until Friday and Donny Osmond covering his Sunday slot again).
Yes, Sara Cox is abysmal. And the powers that be have taken Huey Morgan and Bob Harris off their Friday and Saturday midnight shows, and replaced them with the duff, boring Alex Lester, who is not remotely interested in the music - just plays all the playlist stuff you hear during the day, chosen, no doubt by his producer.