One problem is that the luggage racks could also often be bigger and stronger, but aren't. Some designers of trains have overestimated how much headroom a passenger needs to get in and out of their seat. I concur that the other problem is a lack of space for luggage near vestibules.
If train floors were consistently kept clean (ha!) and more and more airline-style rows of seating are used, it might be an idea to have signs advising passengers to place smaller items of luggage under their seats, and slightly more space could be given for legroom by spacing the rows appropriately, in order that the passenger behind you doesn't have their feet on your luggage.
The other solution is to use more signage to remind people that there is space between table seats, as mentioned above. There are signs to this effect on at least some 313s or 319s (I forget which), where they have rows of seats facing each other, with space in between the back-to-back rows.
377/2 and 377/4 units are, IIRC, the subclasses that are the best for luggage space (when you're travelling to and from Gatwick, that is). I don't find 442s have sufficient space. I've seen many instances of suitcases blocking the aisles due to the racks being full. I could also list a number of other complaints regarding 442s, but you'd be reading this post until tomorrow afternoon.
