Approach the Agency,
HAVING read John Bailey’s article on his roach dilemma, I would consider the following…
Initially, speak to the local EA fisheries team regarding the roach, confirming your understanding that they originally came from the river and were introduced into the pond where they have been fed and subsequently matured.
You may get some questions about who transferred the fish originally, which may be difficult to answer, as that could lead to an historic prosecution if adequate
Some readers would like to see the Avon Roach Project rolled out nationwide, to help revive river stocks of the species. evidence was available, and if a tenacious EA officer had time to unravel the mystery.
It may be difficult to claim no knowledge of who put them in there, given that they have been fed regularly.
If the above is too difficult to manage, you could merely speak to the EA and explain that these roach need to be relocated (preferably into the river, to help future fish stocks). Maybe they could electro-net the pond, remove fish stocks to a suitable holding pond, where they could be tested to ensure that they are free from disease/ parasites that could harm the rivers stocks/ecology, prior to restocking.
If the pond were to suffer low oxygen or a similar catastrophe, where a fish rescue was required, the EA would be the organisation that would save the fish, potentially, so you are merely asking for help without the need of an emergency fish rescue.
I think it might be difficult to transfer the fish without some professional help.
There is also the possibility that a fish farm may be able to help in netting and removal of fish to a suitable holding pond until the fish are certified healthy. A deal could then