There's been quite a lot of research into how environments affect perception and behaviour; it's significant, far more than the average person realises. I think the Thameslink Class 700 interiors are some of the worst on the network and encourage bad-behaviour due to their design.
All the bright white plain/blank surfaces, the blue details, the cold light hue. It creates quite a clinical feel, they remind me of sitting in hospital waiting room. It's not at all 'cozy', you wouldn't want to spend all day sat on one reading for example. This is often done intentionally in some urban design to discourage waiting around, for example in train stations where you want people to move through quickly; I don't really see the design need for this on trains though, if this has been done intentionally then it's quite sadistic. The flip side of this is that it makes people feel like this isn't their home; and is therefore more 'game' to commit antisocial behaviour, such as litter, graffiti, etc.
Compare this to the Elizabeth Line 345s. They're far warmer environments. The dark grey panels and the purple moquettes, the warmer light hue, and the colourful TfL advertising on the walls combine to create a much more welcoming environment. Unlike a hospital waiting room, it could be a modern library or a cool coffee shop. It's much 'nicer'. I'm sure most people would agree they'd far rather spend a day sat reading on a 345 than a 700. It almost feels a bit home-like. As a result it makes individuals much less likely to misuse the space.
I'd love to run an experiment and run both stock simultaneously on both lines and then compare the reports of littering/anti-social behaviour on both. I'd be very very surprised if the 700 didn't have statistically-significant higher levels.