The only area where Swanwick Mil offer a service (unless by prior agreement) is the Pennine Radar area
This is not quite correct. They offer radar services outside CAS to EGBB inbounds and outbounds from the South and West across the Shawbury Triangle. To jet aircraft, this service is generally offered outside of 0900-1700 on weekdays, H24 at weekends. It is quite a popular shortcut, and may be the one being referred to.
LTP