Hi Sue
There are a huge number of variations (well, five)of the 777 aircraft, see
this page.
But, if I've worked it out correctly, your flight is served by a LGW, three class 777 which should mean you are guaranteed AVOD. I say should as of course anything could happen in terms of aircraft substitutions.
WT+ in the 3 class configuration is 2-4-2 (versus the 3-3-3 in WT). There are just three rows in between Club and WT (rows 12,13 and 14). All seats are much of a muchness. No dedicated loos or galley, but this can make for a quieter cabin.
As already stated by others, VS PE is a sort of J minus product (more so out of LHR), whereas WT+ is basically just a bigger seat. Same economy menu, etc. Where they are the same price it would be VS for me anytime, even on the LGW/MAN fleet. But if the BA fare is much less then of course its worth consideration. TBH though, I've never sat in any of the WT+ seats I've ever booked, as I've used Miles for upgrade to get into Club (just 25k return).