If it's a speed for delay then personally I often just say 'when able to make the level restrictions, bring it back to 250kts'. Bit trickier if you're in the middle of a stream of inbounds however.
Expect to be given 250kts due to Gatwick delays an awful lot more in the future. I believe there are plans to bring in the same system as is currently used at Heathrow where aircraft are slowed down in the descent in order to minimise holding. Whether the adjacent ANSP's will be slowing Gatwick traffic down in the cruise for delay as they currently do for Heathrow I'm not sure, however definitely expect it on first contact with London soon.
P.S. I'd love a jumpseat ride in a 787!
Conversely, have you ever been down to Swanwick to see how it works from the other end? Definitely worth a visit...