Typical apartment door locks are very easy to change yourself, at least the kind that's most common here. Requires some relatively simple measurements and screwing in/out a few screws. Hiring someone to do it costs ~150-200€ (130-175£), lock excluded.

This guide has an illustration: https://www.obi.de/magazin/bauen/tuerschloss-wechseln (text is German but the pictures show everything)

Perhaps in the UK different kinds of locks that are harder to change are more common?