Scuzi, you are right 5nm and 3nm miles depending on where you are in the London TMA and which radar source is being used.
2.5 miles can be used on base leg and final, when conditions support it. Strong winds = lower ground speed = lower landing rate. Provided the weather is suitable, the tower and radar controllers agree, 2.5 miles spacing can be used, between permitted aircraft types, where wake vortex is deemed not to be an issue. By reducing spacing to 2.5 miles, particularly in strong winds, we are able to sustain the landing rate.
Wake vortex spacing itself is another issue. Generally, we adopt the ICAO standard recommended spacing, with one or two differences. CAA are currently reviewing UK policy with regard to spacings, A380 concerns constantly change!
Fatfish, why do you ask the question, have you had issues?
2.5