I've noted many Police forces are misinterpreting or appear to be ignorant of the substance of the Coronavirus Regulations 2020. They seem to have got their information from political announcements rather than the Act itself.

I've read the Act and nowhere does it state you cannot use your car to drive to a safe location to take any form of exercise nor does it limit the duration, type or frequency of that exercise. It does not state it must start from your own home

Its states "During the emergency period, no person may leave the place where they are living without reasonable excuse." Note those last 3 words whch the police appear to have changed to "unless essential". Its worth spending a few minutes to familiarise yourself with the new law

read it for yourself

http://www.legislation.gov.uk/uksi/2020/350/regulation/6/made

Its not my intent to attack the police as they are trying to get people to use common sense which more often than not recently appears in short supply. However when they interpret the law for us they cannot change its wording to suit themselves. That is not their role.
Stay safe.