I'm on my phone so can't see your stats as to where you are in your journey.
Some might advise you just go with the flow and carry on blithely afterwards but, in reality, it's never quite as easy as that and often breaking the momentum is a big mistake and the end of one's diet.
Can you reschedule the cream tea? If not prepaid, reorganise the occasion to go elsewhere for a steak and salad type meal?
Good luck whichever way you go...
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