Radar services available in class G in the south east of England mitigate the risks to a great extent, but total reliance on TCAS is a problem due to the number of aircraft operating quite legally without transponders or RTF contact. I'm always conscious that compliance with (for instance) a TCAS RA climb instruction might take you into conflict with a non-transponding aircraft above you (and hence not detected) , so I would try to get you to route clear of other radar contacts even under RIS.