Reckon this is a bit of an odd one ... no right or wrong answer.
On one hand you have people who 'need' everything taking care of. The thought of having to navigate, carry gear, sort out somewhere to stay overnight, etc just doesn't appeal. For those folk, paying £370 would seem a small price to pay to remove any perceived hassle. As SR says, for some there's also the draw of the 'event' and taking part.
The other hand holds (I imagine) many of the folk on here. Navigating, carrying gear and sleeping wherever, are actually plus points, so it's hard to attach any value to them ... in fact we sometimes pay to do them (although it's a tiny sum compared to £370). I think once you're caught up in the world of ITT / bikepacking / self supported it becomes easy to forget that much of the cycling world (or maybe just the world in general) are actually pretty soft ... I'd bet that everyone here knows at least 3 cyclists who think you're a mentalist
My own personal opinion is that taking part in something like the crossing would actually reduce my enjoyment of the ride / route due to the supported nature but it's not in my nature to accept assistance or generally ask for help. £370 does seem like a lot of money but there's plenty of people out there who would consider spending £150 - £250 on a piece of Cuben fibre insanity
I do wonder whether we've maybe become a little 'spoilt' with our underground, grassroots, back to basics, type events?