Top level PAS guys. If you intend to work beyond 55, leaving a few days before your 55th birthday makes a lot of sense. It will reduce your pension by about £10000 pa....which means that you'll only lose £5800ish after tax and NI. That's£58000 over ten years. In return you get another £110k gratuity at 65. Assuming inflation is 3% that means it's true value when you receive it will be about £80000. So that's just over £20000 extra in your pocket if you live to be 65. And thereafter the benefits increase over the guy who stayed to 55 because you can generate an extra £4000 a year from your additional gratuity.
For the less well paid it will still work if your post-retirement income takes you well into the 40% tax bracket.