Given the circumstances, Motability will most likely grant an early termination. You will have to pay a £250 fee and won’t get any good condition bonus, however you will get a pro rata refund of your advance payment (but not any optional extras that you may have added) which you could use towards a replacement car. Advance payments are very high these days, but will only get worse from July once the government imposes VAT on advance payments and insurance premium tax on Motability customers, plus lease terms are likely to become less attractive (lower mileage limits etc), so it might be better to bite the bullet now if you can and get ahead of the adverse changes.
Will this apply where it is a named driver with the problem rather than Motability Customer? Mention purely to help ensure the termination request goes well.