London Branch of John Lewis
John Lewis on Oxford St have a wooden box representing cabin baggage maximums. I spent a fun 15 mins trying to squeeze a variety of different bags in to the box. in the end i settled for quite a big black samsonite which i didnt think would fit but somehow did. the wheels are tucked in well and dont pretrude.
Definitely worth going down to the store to check it out. I think it's on the 4th or 5th floor travel section. My advice is ignore the staff as they dont know what actually fits in the box anyway and were surprised when my bag did.
I love travelling hand luggage only