They changed to 2,400ft intially just after I retired back in 2008; dunno why because the system we used before that was to vector the IFR inbound at 1,900ft and keep any transits at 2,400ft passing traffic information to both aircraft so you didn't then have to try to 'control' a VFR transit to enable you to descend the IFR inbound through it when it was on the ILS.
I can understand using 2,500ft on 06 now they have CAS in order to stay above Odiham traffic, but why 2000ft on 24 I don't know especially as the base of their CTA is 2,000ft just south of Fairoaks and they're supposed to keep IFR traffic at least 500ft above the base.