Whether you fly the B747-400, A340 or A330 as things stand at the moment you would be LHR based...
Whether you actually fly from there is another matter!!!
The B747-400 is operated from LHR, LGW, MAN and GLA
The A340-300/600 is operated from LHR
The A330-300 will be (today's plan) from LGW, MAN
As to what new recruits go onto?? well who knows but suspect the A330... so i hope you enjoy MCO as you will see an awful lot of the place in the first year or 2!!!!!
hope that helps
FC