Well, my tap air flights haven't been cancelled yet, though that may change who knows. Now that Lockdown 2.0 has officially started, I can see that carriers like ryaniar are still accepting bookings to fly out even this weekend...
From my research, it appears the government themselves are not cancelling flights in the UK/Portgual. Rather they are placing restrictions on travelling on the passengers themselves which may cause many passengers to cancel their flights. With less passengers flying the airlines may cancel some flights so that they operate on a reduced frequency. This may mean they move your flight around 1/2 days. However, mine is still on track. Even then, if many passengers are still flying the airline may not cancel any flights. My sister just sent me pictures this morning of the the terminal for Air Vietnam full to the brim with passengers... So they're clearly not cancelling their flights.
In terms of entry, I think it depends, but I think porto counts as mainland portugal which according to the below means I can still enter fine as I am UK national. There's a chance they may conduct a screen on arriving at porto airport but I'm not sure. Dr Ferreira says he will conduct a covid screen for me the day before the surgery though if they don't do one for me beforehand though at the airport etc
https://imigrante.sef.pt/en/covid-19/faqs/#1596230931926-35f82bfa-7209
In terms of restrictions from leaving the UK, the government only advises against all but essential travel. However, they also say what is 'essential' is not legally defined and that they leave it up to your common sense such as health, education or work. I will take the health route and just show them my medical forms with the Dr confirming my surgery date if they ask. I may actually ask him to write me a letter confirming I am in the country only for surgery and not to 'holiday'. Indeed I don't plan to be holidaying but stuck in work, which I can continue to do as a remote worker.