The U2 interceptions (Exercise Trumpet) were carried out by AFDS using two F1As out of Middleton St George (now Teeside airport). Special clearance was given for supersonic flight over land as long as it was outwith 25 miles of built up areas.
Rules of engagement were strict; interceptions were limited to visual ident passes, and if visual contact was not made by 5 nm the attack had to be broken off. The interceptors were not to approach closer than 5000ft astern and under no circumstances were they to pass in front. The pilots wore the Mk5 anti-g suit, a Taylor-Baxter pressure helmet and long sleeved partial pressure jerkin.
Intercepts were initially carried out at 60,000ft then at 65,000ft. Further trials were then carried out above that, the highest being at 68,700ft.
On 7th Aug 1979 Brian Carroll managed an indicated 87,300ft in an F53 over Saudi Arabia. Flt Dave Roome of 74 Squadron saw just short of 88,000ft in an F6 while carrying out a PI on a USAF RB-57F in the late 60s while performing high altitude trials out of Tengah.
Source; Lightning from the Cockpit by Peter Caygill.
An interesting book with one glaring ommission. None of the pictures or diagrams show the interior of a Lightning cockpit!