Just announced today and and saw at my local store.
Starbucks are now offering FREE WIFI to all holders of its Starbucks reward card. Its free to sign up for the card, just go into any Starbucks store and load it with an initial £5.
After registering your Starbucks card online, you can then go into any Starbucks, select the BT Openzone network, and click the rewards card logo on the landing page, and enter in your reward card username + password. No purchase necessary!
Good for those who travel around alot but don't want to pay the exorbitant wifi subscriptions. Also there is a Starbucks in literally every airport, train station, street corner etc so quite easy to come across one of its 650+ stores in the UK.
The reward card also gives you free espresso shots in your drink, free syrups, and a free drink of your choice if you buy a bag of coffee.
Note: I wanted to link to the official page on Starbucks site about the free wifi, but at the moment it incorrectly states that it is only available at 4 stores in the UK (http://starbucks.co.uk/en-GB/_Card/Starbucks+Card+Rewards+Wi-Fi+Support.htm)
Confirmation of nationwide rollout by Starbucks PR: http://www.pcadvisor.co.uk/news/index.cfm?newsid=3202368