Had something similar recently. Still in the cruise and informed delays into LGW. Asked to slow down to 250 and to expect some holding. Replied we would slow down immediately to min speed and then 250 when able. Controller was ok with that.
Saved about 5 minutes with the speed reduction but still given the opportunity to admire the Sussex countryside for about ten minutes! Pretty rare to hold but from comments above perhaps it'll become the norm. As an aside always a pleasure to work U.K. ATC. Thanks guys.
It is very common for traffic inbound to LHR to be given a speed reduction some way out if there are holding delays. It is co-ordinated with neighbouring ATC units so you could be asked to slow down by Scottish, Maastricht, Brest etc. A Mach number reduction of .04 in the cruise and 250 kts in the descent seems to be the norm.