I'm having some trouble parsing this. A lot of musical instruments are larger than 1m in one direction, but can be carried by one person without assistance (even a guitar ...). I presume these are permitted but it's not awfully clear. It's even less clear when you consider a BBb tuba - yes, I can carry it (but not too far), and it lives in a case with wheels on (which makes it a reasonably easy item to drag around). It'll take up a lot of room in the vestibule!
I once saw someone buying a ticket at Waterloo with two cellos, but too her credit she is carrying both of them!
https://www.flickr.com/photos/30661871@N03/27162019624/in/photolist-Hods8b