To go along with the spirit of the question, which is to buy something indulgent, rather than feeding the kids or whatever, so here are my choices
1. Fully equip a machine shop: Bridgeport or ideally Aciera mill, a suitable lathe, pillar drill, maybe universal grinder, maybe foundry set up too. OK I don't have the space, but we'll gloss over that as it's a fantasy purchase. £10k would get me the lot (2nd hand obviously) plus some tooling
Or, option 2:
A classic motorcycle: either Ariel square 4 (might need a top up from my own money, but there or thereabouts) or failing that, a nice Velocette which would be just shy of the £10k. Again same constraint as option 1, as I'd not want such a steed to live on the street l, nor would I want to mend it on the street.