I am not sure there is an alternative provision, since the pizza is vegetarian, and that's the limit of where this goes. Welcome to Flyertalk and welcome to the BA forum, it's great to see you here, Papa.Lima.Echo, and I hope we will see more of you. One of the really good things about this forum is that somewhere almost every specific can be found, and as it happens xenole has thoughtfully photographed the ingredients to the pizza, and it can be found here (along with other menu information):

https://www.flyertalk.com/forum/brit...l#post30759467

So to be honest at WTP level you should consider ordering the GF meal (likely to be unexciting), and / or bring a few GF options of your own along too.