The British police have a tendency to perform welfare checks on elderly men they find walking around in the street in just their underwear.
The welfare check may lead to the geriatric ward of the local hospital.
Should the police stop you for a welfare check they can detain you even if you haven't broken the law if they feel concerned about your mental health and things rapidly become complicated.
Even if you are not breaking the laws of the land you may be breaking local bye laws. I know of French towns that allow speedos on the beach, on the promenade, on the road alongside the promenade but not up the side streets beyond.
Padded Lycra cycle shorts are much more comfortable on a bike and would be my choice. You can get square cut padded Lycra cycle shorts intended to be worn under other shorts but which look presentable on their own. I have done so but only on cycle routes near the coast.
https://www.ebay.co.uk/itm/363775956...AOSwY2tiGLe A
The men's bikini briefs you have are made by Brave Person, I have some and I like them, I'd wear them on quiet beaches and in the dunes beyond, places where anyone I stumble across would not be shocked or surprised by my skimpy attire.