Nearly halfway, yay!
To be honest I would mostly just be astonished because I have never bought a ticket and have no intentions of buying one in the future.
I like to joke that I have already won the lottery (without buying a ticket) because my first paid gig in zoo education was part of the Skills for the Future programme... which is paid for by the Heritage Lottery Fund. So technically I have already won £10k, which isn't much but not bad for zero investment. Of course the more important thing was that it gave me the right skillset and the right CV experience to land a proper job doing something I really want to do. Aww, how sweet.
In the spirit of the post, if I suddenly came into a lot of money unexpectedly... I would spread it around my family, they've all done a lot to support me over the years, and it would be nice to repay some of that investment (unlikely to happen on zoo wages!) If it was a lot of money I'd probably buy a flat, maybe a small house, and then find myself a good financial advisor and invest the rest.
If possible, I'd probably like to cut back to part-time work (I love my job, but why work full-time if you don't have to?), and I would be using my interest money to take fun holidays mostly, I expect. I might buy myself a fancy new bike, too.