The problem with parking 2 747s on G is it would block G for aircraft from the southside. We now operate a one way system southside, so aircraft either enter the runway at H and vacate at G or vice versa depending on the runway in use. Maybe also because it could be perceived as less secure, harder to see over to G than E.
Six nations isn't at Cardiff until 21st March, I'm sure they will have gone by then.
Also G is used for remote parking for aircraft, So parking aircraft on G would mean EGFF would be limited if we needed to park something away from the terminal.