Cat C and D no longer exist. Its now S (structural) and N (non structural).

Its harder for someone else's insurers to write your car off, as they have a duty to return you, the victim of their customer, to the position you enjoyed before then collision, or else provide you with the financial wherewithall to restore you to the position you enjoyed prior to the incident. They will inevitably try it on, but theyre lawfully obliged to compensate you in full. A valuation from a DVLA recognised Ford club might help your case.

If you don't get your car fixed, or given the cash to buy another of equivalent value/quality you can raise a complaint with the financial services ombudsman, and they uphold over 50% of cases so if your claim is reasonable and justifiable you've a good chance.

Unless you've some emotional attachment to the vehicle, or it has some interesting provenance (it was once owned by Princess Di etc) its not worth the effort of buying it back unless you can get an N certificate, but even then you'll lose out if you ever sell it.

Glad you're OK though, that's the important thing.