http://www.gov.uk gives these sorts of Bank Holidays as 'Christmas Day (substitute day)' or 'Boxing Day (substitute day)'
Does it really matter what these Bank Holidays are called? Historically the 27th might have been called Boxing Day but the modern usage is to call the 26th December always as Boxing Day. Old farts might very well differ but they will eventually fade away and die out. It's what old farts do.