As far as I know, it is already that way in a few of their branches, but the last one I went to gave you a voucher for some free WIFI time with a purchase. When the time ran out, you were compelled to buy another brew. I think that is basically the norm. You don't need to sign up to the loyalty card or anything. When I was in Bahrain, and more recently in Mexico, it was a similar setup at Starbucks branches over there.
I haven't had to time to read it completely, but this recent article tells you some more details about plans for the UK.
Free Starbucks WiFi in the UK? Don’t hold your breath
However, one thing I can say about the UK, (at least compared to Spain), is that there is a LOT more open wifi floating about in the air. I am in the UK right now, and am posting this from an unsecured wifi network. I had another exceedingly fast connection in the hotel I stayed at 2 nights ago! Ironically, the hotel was also offering a ridiculously overpriced PAID wifi option. My advice is never get one of those until you have scanned for signals in your room first!
