The Youth Hostels Association has a website dedicated to its 100+ properties which are only available on an 'exclusive use' basis. (You don't have to fit into the 'youth' category, and you don't have to be YHA members). You should be able to find something suitable for your needs: http://www.escape-to.co.uk/
The Confluence Centre in Kilnsey, North Yorksire is well-equipped, has plenty separate showers and toilets and twin bedrooms, and a good kitchen. There is plenty of communal space both indoors and outdoors, ample parking and the most wonderful walks from the doorstep.
Google Confluence Centre, Kilnsey for details.
If you try it, I'm sure that you and your friends will come away with as many happy memories as I have over the years.