The exemption section of the Statutory Instrument referenced by
Tobias-UK above includes these exceptions, where someone has a reasonable excuse not to wear face coverings. Note that BA is at liberty to create its own exceptions in this area, plus the SI essentially covers England's (not the UK's) airspace. But I suspect this falls into the "this will do" category.
Originally Posted by Statutory Instrument as referenced
4. For the purposes of regulation 3(1), the circumstances in which a person (“P”) has a reasonable excuse include those where—
(a)P cannot put on, wear or remove a face covering—
(i)because of any physical or mental illness or impairment, or disability (within the meaning of section 6 of the Equality Act 2010), or
(ii)without severe distress;
(b)P is travelling with, or providing assistance to, another person (“B”) and B relies on lip reading to communicate with P;
(c)P removes their face covering to avoid harm or injury, or the risk of harm or injury, to themselves or others;
(d)P is travelling to avoid injury, or to escape a risk of harm, and does not have a face covering with them;
(e)if it is reasonably necessary for P to eat or drink, P removes their face covering to eat or drink;
(f)P has to remove their face covering to take medication;
(g)a relevant person requests that P remove their face covering.
And the relevant extract from the Equality Act on disability says:
(1)A person (P) has a disability if—
(a)P has a physical or mental impairment, and
(b)the impairment has a substantial and long-term adverse effect on P's ability to carry out normal day-to-day activities.