What about professional debt collector company? If all proofs are valid, just contact with them.
I used to work for one of the largest in the world, but unfortunately I am not resident in the UK.
They don't have office in UK, but they collect debts worldwide:
https://www.intrum.com/en/services/collection-services/worldwide-debt-collection/
They also have special fundation that can buy debts:
https://www.intrum.com/en/services/collection-services/debt-purchase/