Main meal preordering is only available on flights from the UK. (Are they available on all ex UK flights now? I know initially they weren't.)

On flights to the UK you have to take your chances in the onoboard meal choice lottery. If your wife needs to guarantee gluten free, I think she should order a gluten free specia meal. Or bring some food on board that she can eat if it turns out there is nothing suitable on the regular menu.