As always, the
Sherpa website is a very useful tool, although anything it says should be double checked with the FCDO website and relevant country's embassy just to be sure.
It shows that 11 countries remain entirely closed to vaccinated British citizens:
There may be a few more, as they don't hold data for 25 countries.
The following 68 countries require some degree of testing for vaccinated British citizens - though not all require PCR tests:
Apart from the 5 countries which require quarantine upon arrival:
all other 124 countries permit entry without testing, although the degree of restrictions (forms etc.) varies significantly.
Certainly amongst most European countries, there are now no longer any formalities beyond showing your vaccination certificate.