So are you looking for internal upgrades, or just external parts like the rail system, since you mentioned AirsoftEd's gun?
As Duck said I think I'd just leave it because performance should be good already, if not fix whatever is wrong then when it happens.
Never do any internal work before myself, even though I wouldn't really consider myself a noob I have been really bad at looking after mine - reluctant to do it thinking I would break them.
If you really wanted to improve it, would send it to local tech / retailer to do the work. Would say get a TBB, new hop up and bucking. Madbull 6.03 TBB 509mm could be bought for around £33, hop up for example a ProWin but I'm not sure which type the L85 takes...make £20 depending on if you want it from a UK or Asia store.
There are loads of brands of hop up buckings but Tokyo Marui, G&G, Lonex or a 'Prommy Purple' (http://shop.ehobbyasia.com/prometheus-air-seal-chamber-hop-up-packing-soft-type.html#.VWsC6M9VhBd) are all good.
External upgrades for an Afghan-spec A2 would be the Daniel Defense RIS ~ £100 for a Madbull version or can probably even get a real steel one for that, seen em go for less in fact.
A Grip-Pod, airsoft clones are about £10.
Magpul style rail ladder covers, again about £10.
4x32 Trijicon ACOG (not sure of the exact model) anywhere from £40, and a Docter RDS / Elcan Specter DR. You also need a specific riser for the former, but I don't know the name of it.
LLM-01 Replica
'Vortex' Flash Hider?
E-MAGs
I think G&G and ICS both sell Picatinny rails for the L85A2 as well to replace the standard Dovetail one.