With the caveat that I don't have first hand experience of EGLL ops... I do know they are usually running at pretty well max capacity. Look back to last weekend and storm Ciara....regardless of increased spacing, how many aircraft went around off their first approach due windshear/turbulence? If 25% of inbounds made two approaches (or even more), then instead of (say) 40 arrivals per hour, you suddenly have demand of 50 per hour for the same number of aircraft. I can see how you'd very rapidly run out of capacity, then of course there are all the ones which may have had to divert elsewhere and eventually find their way back to base. I suspect that by proactively cancelling quite a few in advance, an airline can recover back to normal ops much quicker (rather than having to bus passengers around the UK / position empty aircraft back home etc) and presumably the overall disruption to the airline and its passengers will be lessened.
This holds true for any airport and any airline, obviously the quieter the operation, the more natural slack there is in the system.