A 'common' UK SOP is, as 'Cobbler' says, to declare a 'PAN' if you MAY land with less than reserves. A MAYDAY is commonly reserved for you WILL land with less than reserves.
The CAA 'Pink' that 'Daysleeper' refers to does advise ALL aircraft inbound to LHR/LGW and STN to have AT LEAST 20 minutes holding fuel. At 0700Z I would certainly try very hard to have at least that.
The issue of nominating LHR as a LGW alternate was one we fought with in BA shorthaul having also been told we were not expected to go there for real.....................
Edit for Blackmail - Case 2 is not normal procedure in the UK. Telling ATC you are committed to LGW will do nothing (in theory). There is a chance the runway can be blocked by an evacuation or tyre burst. Case 3 is not applicable in the UK - in both cases, in the UK, it is Pan or Mayday or just get on with it.