I agree with you. BUT in case of emergency, a blanket ban on something is far easier to understand for the masses. If people are confused what they should be doing to the extent it is causing risk to life, simpler just to ban going outside so confusion is lost.
To their credit UKGov is doing their best NOT to enforce stricter measures, unlike our EU friends. But their hand may be forced.
Of course, sunbathing a few metres from anyone else is not dangerous but the issue is everyone congregates. Find somewhere isolated and you won't upset anyone.