Is carrying 17” backpack ok for most London attractions?

It will depend. Some locations especially museums prohibit bags like backpacks and even large shoulder bags.

I would expect that most places will expect you to check it with the cloakroom, or put it in a locker, or whatever the specific location does for people wanting to check coats, or they may turn you away. Especially museums. When you swing around, priceless items could be sent flying.

On the other hand if you take one to a match at Lords as long as it passes interior inspection you'll probably be fine.

You could check each location's website.

most attractions is a broad target. London has hundreds....