Jamienbaker,
The crappy system is a marconi system called G-Mis.It's on most of GE powered 777's. G-ZZZA-C and G-VIIA-S with exception of O,P & R (which are LGW based anyway)

The newer system is a Rockwell Collins system,bigger screens etc,and is fitted on GE powered 777's G-VIIT,U,V,W,X & Y (there is no "Z" for some reason?) and all the RR powered G-YMM's.

No way to know in advance which you will get...I went out to JFK last week on a G-Mis and came back on a Rockwell.........
If you want an idea of what they have been using of late check out this site

LHR-LGW

Look at current movements for LHR then BA ACARs and check back over the last couple of weeks on your Flt number to see what they've been using.
May give you an idea of what to expect.
Hope this helps.
cheers.