The MB, BS or MB, ChB, etc. aren't like other degrees in their classifications.
Often it's classified purely on a pass/fail.
Also, as there is no standardised examination for the pre-clinical years in the UK, the medical schools can set the percentage to pass (say 55%) wherever they like and is dependent on the exam perceived difficulty so the pass percentage varies.
Some universities, such as Imperial, have "Pass (with merit/distinction/distinction*)" on the final certificate which for example could be "Pass with Merit" meaning the candidate attained a pass in the top 30% of that years' cohort of medics.