Unsatisfied service

I have cancelled my insurance with them yesterday they send me a confirmation letter it’s been cancel and i will not have any cover from midnight yesterday one thing i do not understand and not happy i have paid for the full month my next payment was supposed to be months end again this month but they will not keep me covered until month end of August. So they just keep the full amount but I don’t get covered untill month end if so then they must pay back the rest of the money for not being covered for the next twee weeks😡