Boomers only exercise with hunter killers of their own countries. There are no exercise between boomers, especially of different countries. This is a chance encounter. Usually navies of UK and France are very keen on these chance encounters, so as to get "competition" data (I noticed it was more important for many french sub officers to document the audio weaknesses of UK subs, than russian subs...)

The Triomphant was only a couple of days from l'Ile Longue, so we know it was in the Gulf of Gascony. And it was a scheduled return apparently, this is near open-info from the french ministry. The Triomphant was coming back from the north atlantic (because the M45 missiles have a short range, we still need to be there to shoot moscow, old habits die hard).

The Vanguard has longer-range Tridents and should be patrolling a larger zone for better security, so I have no reason to question it's presence so far south of its base (the UK territorial waters in the atlantic are too small to be safe, in contrast to the US waters in the pacific, so UK boomers have to roam the open, whereas US subs keep hiding in their own waters around some archipelago of tiny desert atolls in the aleutians for instance, making collisions and stalking impossible for the russians)

Pierre