All British citizens are entitled to a passport.
Some people have
temporary restrictions placed on overseas travel e.g. the person in question would have been prohibited from going abroad during any licence period at the end of his sentence but, even then, he would have been entitled to obtain, hold or renew his passport during that period.
Similarly, some people with Football Banning Orders are required to hand in their passports to the police for a few days when certain matches are being played overseas but, once again, they are only temporary restrictions and the basic right to hold a passport holds true.
It's up to individual countries to decide whether they'll grant access to someone with a criminal conviction but all EU countries are automatically obliged to grant entry to all EU citizens (with very limited exceptions, which won't apply here) and there are only a small number of countries (notably the USA, Australia and Canada) where a visa is likely to be refused.
https://www.nacro.org.uk/resettlement-advice-service/support-for-individuals/travelling-abroad-and-immigration-to-the-uk/declaring-your-criminal-record-when-travelling-to-specific-countries/