That's pretty cool, UKMustache! I'd be a little nervous with my first lease being so unusual, but it all sounds pretty safe and legit to me. Ordinarily, we are very leery about proposals to pay rent in advance - in the US, this introduces a major obstacle to eviction. But in this case, it seems that's really not the issue. The tenant is offering it in consideration of skipping the credit check, but it sounds like that's clearly not an important verification in this case.
I'm not sure I'd raise the prospect of crime for the extra deposit issue either. That seems a bit creepy/morbid - your tenant seems safely "collectible" if he's liable for any damages, and I would expect your own property insurance to be the main party in an example like that. Asking for an additional deposit may be fine, especially as it seems he won't have any worries about getting it back appropriately.
A full 12 month term is a very reasonable and sensible demand. Not just for minimizing your tenant turnover, but as you say, winter vacancies can be very slow to fill. If 6 month occupancy is really all he wants, I'd consider offering that but at something like the cost of 7 months.
Wealthy people can still damage property or simply fail to pay debts, so there's some risk even here. But you won't find any tenant, ever, without accepting some risk. Your description sounds like a great option.