I have used a tote style Longchamp Le Pliage bag. I used the medium size when we went to London, but I also got on clearance another style from Nordstrom. This tote expands by unzipping it and then when I arrive I zipit back to its smaller size and have used it as a large day purse.
This next trip I am going to use the Ori Bantry bag (medium) I bought at LHR last year. It will be interesting to see if I like a tote vs. a backpack.
Here is a link to that expandable bag. I paid a bit less than this and mine is in a mustard/tan color, but this is the model. I believe it is too large for international travel. On my first post (edited here) I misremembered and realized I took the expandable one not to London, but on an Amtrak trip. BTW Cheers for Amtrak and their less than strict luggage rules!
https://www.saksoff5th.com/product/longchamp-le-pliage-club-expandable-travel-bag-0400014677222.html