Sorry to hear about this, it sounds really difficult.

BA provides a detailed description of the critieria each of its Special Meals fulfil on this page on its website: https://www.britishairways.com/en-gb.../special-meals

I wonder if one of the vegetarian or vegan options might be best? If it does come with something you can't eat it should at least be separable. You may also want to take some of your own food on board in case it's more inedible than expected.