Just to clear this one up - a lot of the legislation hinges on the concept of "reasonable excuse", so someone living in London going to visit a friend to play dominoes isn't going to cut it as reasonable unless there is some other factor involved such as childcare. Separately there is legislation that allows people to stay with friends or relatives to serve their self isolation period, and if that involves some countries everyone in the household has to self isolate along with the new arrival. So both spirit and letter of law allows someone overseas to stay with relatives. Border Force can refuse admission if they suspect other things come into play which are not legal.

There was a newspaper article I saw a few days ago that points out that while there is a strong tradition in the UK to work within the law, sometimes one's own perception of the law, sometimes one's percaption of what is reasonable within the law, this is actually the wrong way around. The focus should be on reducing social contacts of any sort whatsoever, since that is to a huge percentage the way the virus spreads. Legal context is exactly that, and a virus was never stopped by Statutory Instrument.