Other airlines have prohibited me from sitting on the floor for part of a flight, because I could not reach an oxygen mask from where I was sitting (I had a seat, but felt like sitting on the floor for some different rest).
Exact ops manual procedures may vary between airlines, but if "no ready access to an oxygen mask while seated" is the reason I couldn't sit on this other carrier, then TUI are probably in hot water with the CAA.
Yes, of course you're not near an oxygen mask when you're moving about the cabin to go to the lavatory or whatever, but you shouldn't be moving around for too long, and once up and moving you can keep moving towards a spare mask if there's a loss of pressurisation. The lav, of course, has masks.The issue at hand is the length of time you're exposed to the risk of being more than an arm's length from a mask.
Here's a link to a non-Daily-Mail source:
https://www.bbc.co.uk/news/uk-england-46858249