The issue seems to have been with the radar or radars serving both EGLL and EGKK. I'm not a TC bod but would presume in that case they would have to use another radar head or combination of, to provide a service but due to it being the "back up" that they had to reduce capacity due to a reduction in coverage from the normal feeds. Its a failure that's probably felt more in the LTMA than else where.
Capital investment in infrastructure takes time especially when your customers want you reduce your costs year on year at the same time they want reduced delays, increased redundancy in the system and to increase capacity for the never ending growth in traffic numbers. Especially when NATS has to spend on both new kit and controllers at the same time. (I know which they'd rather spend their money on, and it ain't bums in seats). Problem is kit takes years to develop and adapt to the airspace and controllers take years to train. The CAA need, for once, to say that NATS doesn't need to cut route charges (keep them the same) but instead of paying dividends and making a profit spend it on what it needs to be spent on.
I feel for the people whose flights got cancelled/delayed, its ****. Hopefully I won't be in that situation next week.