have a look at bigholidayhouse.com and groupaccomodation.com. We have used them for groups in the past and found a good selection of cottages. if you are just wanting a weekend then make sure you avoid all school hols because most places will only want to book a week then. lastminute-cottages also have a group website called something like the hen house. Havent used it myself but worth a look. Last girly weekend i did was to Bath. we stayed cheaply in the premier inn place and went to the spa for the day.