It appears that under UK copyright law, since you were not an employee of the customer, the work belongs to your company.
However, the customer has paid your company for the code, so it would be reasonable to conclude that they have a licence to use it. Otherwise, what have they paid you for?
So I don't see how you can stop them using your code. You could ...
The short, realistic, real-world, bottom line is:
there's just nothing you can do.
As others have said, ask a solicitor. But, after you briefly outline the situation, the reply will be:
"Realistically there's literally nothing you can do. To start legal investigations and proceedings will cost some £10,000 merely to get started."
By all means ...
I know this gets said a lot, but you do have to speak to a lawyer.
In my very limited personal experience, you may be able to prove that you have the copyright to your own work, given you've got the repos and emails, and the person who has to have paid you will probably have trouble proving he did it himself.
Whether and how that would translate into getting ...