As all radars are used at Swanwick, I would assume both radars would be available to Heathrow Approach/director etc but they may be 'mosaiced' together.
My own experience on Farnborough LARS North meant I was able to assess both of them and I personally found the Heathrow 10cm had better coverage for the LARS task to north and northwest than the 23 cm, whereas Debden 23 cm was superior to Stansted 10cm for coverage to the north east - you could see traffic almost at ground level at Wyton on the Debden 23 but not the Stansted 10.