As the law currently stands, after 31-Dec-2020 Covid rules in the EU treat UK citizens without a 2nd nationality in the same way as US citizens - i.e. no guaranteed right of entry into the EU - the number of Covid cases in the UK is too high to be treated as a low-risk country like New Zealand
However, it is very unlikely that that all EU members would really want to enforce such a rule. Thus what happens from 01-Jan-2021 is really up for haggling by politicians, along with everything else