Bailiffs have had the legal right to force an entry to your home since 2004.
That applies to fines, fixed penalties enforced by magistrates courts, or income tax arrears.
Peaceful entry only for all other debt.
Koi could be taken if the bailiff can get peaceful entry to the pond - they are allowed to climb over walls, gates and fences. It would be interesting to see if they fall into the category of domestic pet. However, moving fish and keeping them healthy will take organising. They wouldn't be prepared to do it on first visit.
If you have had the bailiffs round and have expensive koi - move them to a friend's pond if possible.
The herons have been very active lately. :)