That's the first I've heard in a long time of any Hilton or Doubletree in the UK only giving Diamonds continental breakfast in the lounge and charging £10 to upgrade to the full buffet in the restaurant. Haven't stayed in every UK Hilton, but have stayed in quite a few. Always given full breakfast in the restaurant up to now. Many years ago, Manchester Airport Hilton used to do this but then they changed their policy. Even when UK Hiltons have Exec lounges serving breakfast, they've always given us the choice of having full breakfast in the restaurant at no extra charge.
Hilton Kensington has this " Diamonds take breakfast in the lounge" policy too , or at least they did when I stayed a few years back.......I had booked the room on a public sector code ( my lady works in the ps ) , this actually includes breakfast but they still insisted it was lounge only, I took it in the lounge but complained to manager on checkout, turned out I was correct and should have been restaurant......they comped the parking .