The only option is probably to get a lockable post fitted. The only other thing is that the number plate will now be searchable on Google. Anyone thinking of buying this car will probably want the reg changed = hassle and expense. If you can post it on Facebook and Youtube, even better.
https://preview.redd.it/53dgvw9wehy81.jpg?width=4000&format=pjpg&auto=webp&s=fe3cb100b08b9b5e1194b8fbf3e07f479aa3c6ac