Sorry yo hear about your misfortune. Presumably £12,500 is what the insurance company is offering you?
Insurers initial write off settlement offer is always low and probably representative of a trade value, rather than a retail price that you’d need to pay to replace your car with one of a similar age, mileage and spec, and IMO there’s always scope to negotiate a higher figure.
If you can find examples of cars similar to yours (similar age, mileage and spec, etc) that are up for sale at higher prices than you’ve been offered, use these in your negotiations with your insurer, but don’t accept their first offer! Hopefully you’d declared your stage2 mods to your insurer; if not then that could influence the way they deal with the settlement of your claim - and not in a good way.