(Sorry for the continued thread drift, just an attempt to put the Typhoon/QRA performance into perspective))
I'm sure one of you fine chaps can do a direct link, until then sorry for the provenance but a replay of the track flown by the Challenger is embedded in this article:
RAF Typhoons break the sound barrier to intercept Dublin-bound private jet | Daily Mail Online
If that is to be believed it entered the UK FIR northwest of Abbeville and then was on a direct'ish route towards Cardigan Bay (so south of London, just north of Bristol. If the loss of comms only happened/was only detected by ATC at the entry into the London FIR then I'm not surprised the Challenger got as far west as it did before it was intercepted from aircraft operating out of Coningsby. TBF that plot doesn't indicate exactly when the Typhoon's got up alongside, but I'd suspect it would have been before the turn towards north that was made near Brecon, perhaps made to keep the formation inside the UK FIR until the situation was resolved.
If folks want all "no comm" aircraft to be intercepted and identified, subsonic or supersonic, at or shortly after the FIR boundary then they'll need to stump up the taxes for lots of QRA aircraft, holding on CAP (i.e. airborne), 24/7.
Anyhow, hopefully back to the thread.