Never in a million years would I tell anyone other than DH. We were having this very same discussion one afternoon and my DM said she would expect me to sort out the family first. When I queried that her response was that she didn't bring me up to be selfish....! I've also got an ex-BF would you can bet your life would come crawling out of the woodwork even all these years later with his hand out (he was like that). So depending on how much I won, I would take time to assess how best to use spend the money and then drip feed so it's not obvious.
I would, however, start at the bottom of Sloane Street and work my way up buying anything I wanted. As I've got a bit of a shopping habit anyway, it would come as no surprise to people if I pitched up with a Chanel bag for instance - I'd explain that I'd been tucking a bit away for a 'big' purchase.
I would like to say that I would tell work I was leaving, work my month's notice and then just leave. However, the temptation to go in, wait until someone upset me (about 5 mins after I get in) and then sweep out in a blaze of glory after calling them all a bunch of W*ers would be overwhelming.
The thing I think winning the Lottery would give me would be Time. I imagine the luxury of not having to cram everything in at weekends - things like booking a hair appointment in the middle of the week ('can you do Thursday at 1pm?' 'yes, that's fine' rather than having to squeeze in on a Saturday), having beauty treatments when I felt like it, or just sitting and reading a book with a coffee in the middle of the afternoon. The museums I could visit, short breaks and longer holidays without having to plead for the time off.....I could go on.....
Of course the chances of all this happening are remote in the extreme, so it's all a bit of a moot point really 