why does Heathrow need so much Class A? ... LAX for example
Chalk and cheese. Many of Heathrow's SIDs have large turns inside the CTR (eg DVR/BPK off 27's and CPT off 09). It also contains the initial parts of the go around tracks from all runways. The space is all used (or at least "reserved" in the event of an engine failure on something climbing out of LHR) so it would be difficult to shrink the boundary by much.
On the other hand, I have never been refused a SVFR transit via BUR/Ascot whether LHR have been on easterlies or westerlies in 20 years of flying around this area. For an SEP, there isn't really an option to fly across the eastern end of the zone.
The only possible room for improvement (imho) would be to pull the zone surface boundaries in around the western ends so that Denham, WW & Fairoaks ATZs were completely outside the zone and perhaps lift the base from SFC to 1500' around the western area at Ascot/Bracknell/Burnham.