I gathered this a couple of weeks back but now starting to lose interest in Foursquare. I will keep my account and might occasionaly check-in but for now it's going to be left behind.

An easy to read listing of all the rewards avaible in the UK for foursquare:

Birmingham

Brighton

Bristol

  • The River Grille Restaurant - BS1 4QF Anyone: Checkin in on Foursquare to The River Grille and receive 20% off the a la Carte menu. Please let your server know before you order!
  • The Bristol Hotel - BS1 4QF Anyone: Checkin to Foursquare to the Bristol Hotel and receive 20% off a la Carte menu in the River Grille. Please let your server know before you order!

London

  • Bea's of Bloomsbury - WC1X 8NW Mayor: Whoever is the mayor of Bea's of Bloomsbury gets a free cupcake! :)
  • Bou Tea - WC2E 7DD Mayor: Mayor drinks for free! Anyone: Check in before 10am, and receive a free pastry with any drink purchase.
  • Debenhams Oxford Street - W1C 1JG Mayor: Free coffee in the Debenhams Cafe! Anyone: Check into Debenhams Oxford Street on a Friday and get a free coffee.
  • Dorset Square - NW1 6QN Anyone: Free glass of prosecco for every 1st check in!
  • Dose Espresso - EC1A 9EJ Mayor: Friday is Free Coffee day for the Mayor! Anyone: Check-in on Foursquare and receive a free coffee with any food purchase of at least £5
  • Galvin at Windows - W1K 1BE Mayor: Anyone who becomes the mayor will get a complimentary bottle of champagne for their birthday as well as a window table overlooking the city from our bar.
  • Hummus Bros. - W1F 0TJ Mayor: Mayor is king at Hummus Bros! Check-in, show that you're mayor, and share the wealth with 2-for-1 on main courses for you & a friend.
  • Kaffeine - W1W 7QJ Mayor: The mayor of kaffeine is welcome to a complimentary double ristretto for each visit when you come in and say to us, 'I'm the mayor and this cafe is not big enough for two of us'!
  • LivingWell Docklands - SE16 5HW Mayor: Show that you're the Mayor of Carrabba's and enjoy a complimentary dessert with the purchase of an entrée. Grazie!
  • Nincomsoup - EC1Y 1BE Mayor: 25% Off for the Mayor!
  • Paul A Young Fine Chocolates - EC3V 3LP | Paul A Young Fine Chocolates - N1 8EA Anyone: Show the store you've checked in on Foursquare and get free award winning chocolate truffles. Best in the U.K.!
  • Watershed - SW19+1SD Mayor: The Mayor on each night gets a bottle of Codorniu Rose free. Anyone: Any Friday & Saturday in February - 11pm to 1am Check in using foursquare when you get to the door at The Watershed - show us - get in free.

This post is out of date.

Doing a quick search I found this site which is pretty good at finding rewards/specials.