While I certainly agree that instrument flying skills, and the ability to manage a cross country flight well are very important, I'm not so quick to agree with
aerobatics and sea plane ratings arnt much use for flying out of Heathrow
Aerobatic skills can be suddenly very useful, and life saving, even at Heathrow (should you find yourself GA flying there). The confidence to regain controlled flight of an upset aircraft is vital (wake turbulence comes to mind). The understanding of what many aircraft can actually do, if you fly them with skill, through unusual attitudes, is very worthwhile.
As for a seaplane rating, I do see that there are perhaps limited opportunities to fly seaplanes far and wide in the UK. However, this does not reduce the value of the skills you gain in understanding the wind, and it's affects on your takeoffs and landings (as well as surface maneuvering). Even more importantly, as the new view on a skill for single engine GA pilots.... The skills learned during some water flying will improve your observation and decision making skills, when it comes to precision approach and landings at places you have selected, which are not runways. This can be a great confidence booster at forced approach time regardless of the undercarraige type.
Any new and different flying skill you can gain, even just different types, will entwine itself in your basic skills, and make you a better pilot. 'Thousand hours, or hour a thousand times.....