they would be liable if they were going to refuse any requests for reroute on other airlines.

i am assuming you would still prefer to come back to the UK on the original date? if so i would call and see what they can offer. there are AF options via CDG which should be offered. there are also U2 flights, but BA cannot offer them - however depending on the price you might be better off if you book U2 yourself and refund the BA flight if the former is less than the latter.

and finally i don't know if there are options from MRS or TLS which would work for you as BA serves both those airports too.