My vote would also be for 121.5 and 7700 for several reasons:
1) The call on 121.5 will be immediately triangulated on D&D 's magic map so you have a head start on being located
2) D&D will always be the first to call out the SAR facilities even if you make your first distress call on someone else's frequency
3) 7700 on the squawk will set (audible) alarms ringing at D&D and also (visually) alert every radar unit within range of your predicament
4) London FIS have no 'panic button' (at least not for this). They will simply call D&D by phone and pass on what you have told them. You can cut out the middle man in this case, which might result in a slightly shorter swimming lesson.
The exception to this is if you are under the CONTROL of an ATC unit. Then you MUST make call on their frequency so they know what you're doing and can move other things out of the way.