Quarantine query

The government's quarantine requirements cover all entrants from the 'forbidden' countries, regardless of individual health conditions. You are correct that there are no certain figures for immunity after previous infection, although there are no proven cases of 'second' infections. [There were suggestions earlier on that there might have been some, but problems with testing mean that it was uncertain whether they had actually recovered before being tested positive again.]

If you have had it, however, I would personally be a lot less worried about contracting it again this year at least.

Whilst you may have had it and unlikely to catch it full blown again, there isn't any evidence either way concerning if you could be a carrier and potentially able to spread it if you've been in contact with someone who's got it currently

So much we still don't know about the virus.