Hi again Erik,
The train service down here (South West) does have some new trains, but also a lot of old rolling stock & it is purely luck (or lack of it) that determines which sort you end up with.
For the coach from London Victoria, allow up to 3 hours, and for the return trip to Heathrow, you are talking of up to 2 hours. Plus 15 minutes for the taxi at this end. So remember to build that into your flight time plans.
Also although it is listed on their website, I have NEVER seen a National Express coach at the cruise ship terminal, the one you are going to get will bring you into Coach Station, Harbour Parade.
Last: Cunard Queen Victoria January 2016
Next: Oceania Nautica Aug 2016
Then: P&O Oceana December 2016
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2), Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey