devsan,
Hello...
I am BA crew and may be able to shed some light on how the seating works, to help you make a more informed decision.
With the greatest of respect, you do sound like a larger than average built chap, and I can see why you might be a little concerned.
If I am completely honest, I don't think World Traveller Plus is going to be best for you. Mainly because, the arm rests are fixed and don't move. So, if you are quite broad around the behind and hips, then it is going to be a bit of a squeeze. Whilst the leg room may help, the width may be something to strongly consider.
Have you considered buying 2 seats in Economy? Not saying, by any means, that you need a whole 2 seats to yourself, however, the arm rests DO move in Economy, and it may prove to be more comfortable than trying to squeeze into a WT+ seat. If you call BA, you can book an extra "comfort" seat, which is the ticket price, minus the APD and some other charge (I don't work in ticketing, so don't fully understand it) so it might be worth giving them a ring to ask about it.
I hope you don't mind me saying, but I would hate someone to spend the money upgrading, to then be uncomfortable with no moveable arm rests.
TS