[MENTION=308196]CosmicGirl[/MENTION]

I'm not sure if there is an official MCT. BA says to allow 'at least 3 hours' but it also says allow 3.30 as well!

But 2.30? I wouldn't touch that with a bargepole

Assume you need to have bags dropped off an hour before due to it being long haul that means 1.30 to deplane at LHR, clear immigration (quicker now if you can use the e-gates), collect bags and then get to where your onward transportation (coach? taxi?) is hopefully waiting to depart immediatly and then get to LGW and then get to the airline desks. No bags you'd have more time but it would still be tight. How much time depends on when your ex LGW flight closes its doors plus you'd still have to clear security.

Yes you will be rebooked in the next available flight but that might not be the next flight.

BTW AA flies into T3 not T4 at LHR ...

If you tell us which flights and airlines are involved you may get more targetted advice rather than generic.