It seems that the last recorded use of a BP percussion pistol being deliberately used with intent to cause harm was 1926. With long arms, it's a bit more difficult, as many shotguns used BP cartridges - generally available here until well into the 1960's, and many shotguns were used in crime before and since.
However, there is no recorded use of a BP rifle of any kind being used in crime in UK since at least 1870.
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