You win the equivalent of £10,000,000

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I'd go public.

@ghost-of-a-duke said
I'd go public.

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@drewnogal said
I’d kidnap your dog.

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I would buy flats and rent them out to families at modest prices (as low as possible without losing the tax privilege on the repairs.)

I'd stick about half in a fixed term savings account, and continue to do so year after year as long as interest rates remain decent.

With the rest I'd give some to family, maybe a tenner each, travel and start some kind of small charity or business, or both.

I would build a large no-kill dog shelter, and then entice you and FMF with part of the winnings to be permanent live-in caretakers. You two can flip for it, to see which of you two will be top dog caretaker.