The RAF were always sensitive about the Lightning's performance, other than vague statements like '2.5 minutes to operational altitude'. Perhaps they didn't want to advertise anything that could provide more insight.
I have a colleague who in his line of work tracked them doing practice interceptions against Concordes down the east cost of England; apparently those and the French Mirage IVs were very tough targets because of their supersonic endurance. The Lightnings would refuel and then climb up over 70,000 ft to try to use ballistics to help, but would often have to break-off without achieving a firing solution; they just didn't have the endurance.
Subsonic ceiling was around 50,000 and absolute ceiling in the high 80s.