Once the UK, leaves the EU halfway house at the end of this year, the right of UK citizens to enter the EU becomes weaker. Since the relaxation of total lockdown in July, EU countries have treated the UK as quasi-EU and thus Brits (unlike other non EU countries) have not been told to get lost when trying to enter EU states. Yes, there may be 2 weeks quarantine, but entry is generally possible. See the EU's policy on Serbians entering the EU since July 2020 for details
It is not clear at this stage how welcoming/hostile the EU intends to be to Brits wishing to enter EU states in 2021 while the pandemic continues and before a vaccine is widely available. Airline bookings between the UK and EU may be subdued until there is political clarity