Weather generally is considered a reason not to pay out EC261, and then you have to consider what else would constitute extraordinary circumstances - things that the airline cannot reasonably predict and/or avoid.

A go around + diversion seems to me to be an example of this, but in reality each case has its own merits, and if it ended up in court, it would be how the court judged the outcome. On a good day your argument (which I guess is that U2 should have gone to LTN in the first instance) may win the day, but I have my doubts: the airline still needs to get people to SEN and it is the time at SEN that matters. Having been "a couple of hours" late anyway, due to fog, plus the diversion, it doesn't make that much difference. The airline would probably argue they did all they reasonably could to get people to SEN but the weather conditions inhibit an arrival within 3 (or whatever) hours.

The caveat on that is that I don't see the precise times in your post, so it's equally tricky to give precise answers on that one. Also the cross referenced material is certainly open to challenge, indeed the technical problems section was overturned by the Supreme Court. It is a draft guideline, both words relevant.