I have researched Egypt and Namibia, however both of those countries would instead prosecute the person there rather than accepting the extradition request and sending the criminal back to the UK.
The United Kingdom does not have extradition treaties with a number of countries, including Afghanistan, Algeria, Iraq, Lebanon, Libya, North Korea, Somalia, and Syria.
Egypt and Namibia are among a very small minority of countries that will try someone criminally for crimes committed in another country. Almost none of the other countries would do so.
This said, ad hoc extraditions of individuals who committed crimes abroad in the absence of an extradition treaty are not unheard of. Nothing prevent Algeria, for example, from deporting a U.K. citizen who is wanted for a serious crime in the U.K. to the U.K. despite not violating any Algerian immigration laws, because Algeria doesn't want that individual and they would like to do a diplomatic favor for the U.K. to redeem at some future date when Algeria requires some assistance.
Likewise, Algeria could trade someone wanted for a crime in Algeria held in the U.K. for someone in Algeria wanted for a crime in the U.K. in an exchange negotiated on a case by case basis.
While it isn't strictly analogous legally, the diplomacy involved in such a one off extradition agreement would be not unlike diplomacy involved in the December 8, 2022 prisoner swap between Russia and U.S., of Brittney Griner, an American woman convicted of a marijuana crime in Russia, for Viktor Bout, a Russian man convicted of illegal arms dealing in the U.S.