It's now a requirement of the Care Quality Commission CQC that every member of staff working in a GP practice has an up-to-date Hepatitis B status. The MPS have prepared a summary of why this is needed, as part of the CQC's Cleanliness and infection control (Outcome 8, Regulation 12) "People are cared for in a clean environment, and are protected from acquiring infections".
If you're a chambers locum or are employed as an agency locum, they'll no doubt otherwise notify every practice on your behalf. Otherwise it's a sign of a well organised locum to provide this in advance of working.
If you're employed by a practice as a salaried GP, you'll be entitled to free immunisation status checks and boosters if they're a good employer. As a locum, unless you can persuade a friendly practice, you'll have to arrange this yourself with your own GP as a private service.
Added to this, the Green Book states:
Healthcare workers in the UK and overseas (including students and trainees): all healthcare workers who may have direct contact with patients’ blood, blood-stained body fluids or tissues, require vaccination. This includes any staff who are at risk of injury from blood contaminated sharp instruments, or of being deliberately injured or bitten by patients.
As a GP, if you're exposed to continuing risk of infection, you should have a single booster dose of vaccine, once only, around five years after primary immunisation. Measurement of anti-HBs levels is not required either before or after this dose.