As the Northolt Watchman radar is not 'owned' by NATS, it cannot be used. I must admit it would be useful, but Stansted and Debden are available (but not at the same time), as are the 10cm (old) and 23cm from Heathrow. What the coverage of the 'new' Heathrow 10cm will be like we don't know; it's only just being/been calibrated.
Farnborough has always had primary and secondary 'feeds' from other radar sites by the way, ever since the first SSR displays in 1981.