I am a big fan of Away’s Bigger Carry on (£235) and 25l Outdoor Convertible backpack (£145; slightly oddly different to the 26l backpack - the 25l is shown an an alternative option on the page for the 45l version).
if I had to have one bag to do the job of both, I’d probably go for the 45l backpack and some substantial packing cubes for anything that needs a bit of extra protection.