begun by Joby Andrews who owns three pubs in the Bristol area. Quite a few pubs have signed up, not just in Bristol.
People who register on-line will receive an e-mail confirming their registration. If they produce this in participating pubs, they'll get a free drink. The only two places offering this locally that I'm aware of are both in Liverpool: Ma Boyles in Water Street, and Constellations in Greenland Street.
As I understand it, this is not illegal as it is not rewarding for people for voting a particular way, but simply for being on the electoral roll. It's probably worth it for a free pint and not getting an £80 fine. The thing is: you've only got three days.