they'll probably knock off a fiver and charge you £25 for doing it! Wait till renewal, insurers these days charge the earth for a policy adjustment mid term.
Guaranteed Hire Car charges doesn't add a great amount to the overall premium, but if you cancel it, the short term cover rate, although only for 2 or 3 months, would equal almost the amount of the premium in the first place, which together with the approximately £25 cost of the Mid Term Adjustment, would probably mean that your monthly premium wouldn't change. Therefore it would be best to leave it in place until your next renewal.