anyone one else flying from the UK would have waited until the thunderstorm had cleared then flown to destination and either night-stopped and returned next day or used discretion to get back.
There seems to be a misapprehension that "discretion" is something that pilots can always pull out of the bag to avoid the disruption and incovenvience that flight crew limitations would otherwise cause. It isn't.
Adding not only a sector, but also additional time to a duty period, ie a hour or two at best or several hours at worst, in situations of this nature is so far outside the acceptable use of "discretion" that the company who authorised/instructed it would risk its AOC and the pilots their licences. That's assuming (a big assumption) that adding the sector and time would have taken them right outside their company's FCL rules. But I would be surprised in FR don't roster crew for a normal day that has very little slack in it.
As well as this, a large reactive delay could have disrupted many hundreds of passengers' journeys that day and possibly the following day, and their interests also count.