TEEEJ
Because they would have been using the time to conduct air refuelling from the VC-10 and also burn up their fuel.
Three and half hours seems a hell of long time to return to base, even if air refuelling was involved. I don't doubt the accuracy of the information I'm just curious of what they would be up to for so long given that they were armed QRA jets.
The radio comms mentioned that they were authorised to fly at M1.2 at 2000ft, which is pretty much flat out. Is it known how long they were travelling at this speed? I assume even the Typhoon would require continuous afterburner to achieve this. I wonder if this was the first supersonic intercept to have taken place over the UK mainland?