Reduced wingtip clearance at Fairoaks is only an issue if you're flying a PC12, Kingair or TBM which I presume you're not; aircraft up to PA46 wingspan should have no problem.
Fairoaks is not inside the London TMA but is inside the Heathrow CTR. The 'concession' to operate in class A airspace being limited to the Fairoaks ATZ plus small fillets not above 1500ft QNH when the HEATHROW VISIBILITY IS 3000M OR MORE; if you go outside this it sets off the 'CAIT' (Controlled Airspace Infringement Tool) at terminal control. This annoys the controllers not only because it wakes them up (only joking) but they have to file a report when this happens too. Basically stay west of the M25 and south of the M3. If departing to the west/north west, keep the Bagshot Mast to your right, but do NOT go direct if off runway 06.
Inbound from Biggin, it's better to call 123.225 although Biggin will tell you to call 125.25. This is because all easterly departures from Fairoaks off runway 06 (and many off runway 24) call 123.225 by local agreement, so you could find yourself opposite direction to a departure near OCK. The border between LARS East and LARS West runs through OCK anyway and when I worked at Farnborough,. I always preferred all traffic working Farnborough east of OCK to be on 123.225, but the powers that be decided otherwise.