Al R,
Regret I cannot answer that one with any degree of accuracy. I certainly cannot recall any ejections within the highest altitudes and, statistically, I am sure the vast majority of all ejections occur at low altitudes.
Moreover, the time spent at the higher altitudes by Lightnings would be a very small number against the overall time that was clocked up by the fleet during its flying life. Hence, the risk of a high altitude ejection would be low. Unfortunately
http://www.ejection-history.org.uk/ has not yet indexed Lightning ejections, so a quick check is not possible. An educated guess only, none above 50K ft and very few above 40K ft.
As you say, given the choice, sensible aircrew would stay with their aircraft hoping to achieve optimum ejection parameters before opting for a kick up the backside.
lm