Only on the MMM forums would that stupid joke I made generate a dozen posts of discussion. I heart you guys. :)
I was admiring the fact that the OP had framed the problem in £, I'd responded with UK tax rate examples and we'd then segued into the intricacies of the US tax code ;-) Maybe if I'd pointed out that the tradesperson might have to charge 20% VAT on the work too, depending on the size of their business, we could have digressed onto the intricacies of sales tax?
Back on topic, company I work for was acquired a couple of months back. All employees had share options, so everyone has an unexpected lump of cash. Company paid for a lot of (good quality) financial advice re pensions, tax, ISAs and other investments and (lots of mathematically minded engineers here) most people have been pretty sensible.
Lady I work with calculated that laser surgery on her eyes would pay back vs the cost of contact lenses pretty quickly and has had the operation, which seems OK. However, she's also bought a new BMW on finance (6% interest rate) in spite of having the cash, because "the dealer threw in 12 months free insurance if you took their finance package."