ATC,
I'm not sure which unit you are station at. But generally it seems to me that Farnborough is at RIS capacity frequently when I fly and is sometimes difficult to get a call in before you need to speak to somebody else anyway. Brize certainly went through a phase of declining FIS/RIS requests and would suggest Lynham, Luton or even London Inf!
I'm referring to weekends. The rare occasion I do flying in the week things are naturally svelt, even the air feel smoother mid-week!