Well they arrested a guy for wearing a jock strap as a mask, and another guy for walking down Oxford Street when he was naked apart from a mask over his wedding tackle.
The exact thing to do depends on the circumstances. But in general...
Invoice the arresting officer for taking your freedom away. When he doesn't pay your fee (500,000 pounds), sue him in court, Queen's Bench, man to man, for keeping your property (your body) from going from point A to point B in any free manner you wish.
What he will do is say that Covid is the reason. Make him prove that Covid exists. Do your research, and bring in expert witnesses like Dr. Andrew Kaufman, who show that there never was any proper isolating of the virus. But if their ex-wit, shows virus isolation, let Kaufman tear it apart bit by bit. You just might be the one who sets us all free from the Covid stupidity.
Other than that, if you are fearful that they will arrest you for simple non-social distancing in lockdown, you might be paranoid?
In the States, the lockdowns are voluntary. Even arrests are based on voluntary compliance (like IRS taxes), so you can get out of them by volunteering out. I mean, they are not Constitutional, so where is the agreement that you made? Show the paperwork where I signed on the line. If they can't do it, sue them for disallowing your right to travel.