John R81 has made a lot of sense on this topic, and Puntosaurus' point on why did PB not divert to say Luton is a good one, given a northern destination. Would be quite interesting to know what the EGGW METAR around the accident time was - anyone know? May well have been fogged in too though I guess. Of course not just cost but the faff of a big airport is often another reason to try to avoid them.
Something that seems missing from (what we know about) the accident flight was what contingency plans were discussed between client and pilot/operator, given the high chance that a landing at the Elstree pick up point would not be possible. Where was the client based? Was Elstree the best place to plan for the pick up? Could Redhill have been considered an option where departure would have been possible in far poorer weather than a landing? Was Luton considered as an alternate? Stansted? Wx info was probably not available at Cranfield early enough. These human factors aspects are important and can greatly add to the risks.
Frankly I doubt PB was really pressured into flying - surely he was too experienced and confident for that. But he would have simply wanted to get the job done and not let his client down, so pushed himself too far.
Pressure, whether from clients, operators or from we pilots ourselves, to get a task done is a risk factor. But it is an inevitable part of flying (and a lot of other things) and to an extent healthy, to encourage us to look at ways of achieving (albeit safely) a flight when conditions are more challenging.
I think something that does come out of this accident is the importance of considering very carefully the alternative options, in advance when on the ground, should the weather compromise or prevent the flight. And these options should be discussed with and ideally agreed by the client, and perhaps even conveyed in writing, by text or email. That might help reduce excess pilot pressure and make clients consider alternatives more carefully and make plans for such eventualities. Obviously I'm sure much of this all happens with good operators anyway, though it appears not have done here and had it occurred, the accident would probably have been avoided.