[timwilky]

Firstly, under British law, you are presumed innocent. You do not need to prove anything. It is upto the police to prove that your bike power exceeds your licence. Therefore it is upto them to dyno your bike and prove to a court that it exceed the max permitted power rating.

As they would be liable for any damage caused in the testing process. It would be very unlikely for the police or their agents to risk testing a bike unless there was good reason to do so such as involved in a fatal etc.