I can’t see what the problem is, they employ you they don’t own you 
,
‘‘Cheating on them’’? 
…If it’s a shower of ■■■■ type firm, then who cares if they are ‘offended’,.they will sure as hell be used to drivers jacking as these type of firms usually have a high driver turnover…for good reason.
I’d book a day’s hol,.AND I would tell them why if it was me.