It's something I'd only tell my immediate family (I have a lot of cousins, uncles, etc that I....uh... just hate. Long, complicated, messy stories) and close friends, for starters. I'm not really one for ostentatious displays of wealth so it's not like I'd be trying to keep it a secret whilst driving a Maserati and posting pictures of my new Miami mansion.
I'd just want to sort out housing for my brothers/sister and mum (and nieces)... put together a savings account for my niece's future... maybe buy a holiday home or two abroad somewhere. I wouldn't know what to do with £100m so I'm only really conceiving this from a like... £5 or 10m perspective
I think the Lottery have the option to allow winners to remain anonymous, and I think I'd take advantage of that. I wouldn't at all want to be in local papers. And it's not the idea of past friends/distant family coming crawling back and pretending we were ever close or anything, I just... don't think it's something that deserves press attention