It's company policy at my TOC and part of their job role for the train guard to keep wheelchair areas accessible.
It would appear we're not alone as to quote XC's policy on the same here -
http://www.crosscountrytrains.co.uk...ssistance/disabled-people's-protection-policy - "We will ensure that any spaces dedicated for wheelchairs are not used for any other purpose and the need to keep these spaces clear is a specific part of our luggage policy."
It may be bloody difficult or nigh on impossible at times, but that's the situation as it is. Having slung 8 or 9 items of luggage around a peak Friday holidaymakers train myself (and put the delay into it in the mean time) I know where you're coming from but you have to deal with it, even if you end up delaying the train - especially if they're all like it. It was definitely the case that when the space was displayed as reserved and it occurred the delay was allocated to the train manager. I will ask briefly for the owners of the luggage to come and help move it, but if they don't appear I've got no hesitation in chucking it about myself - that said I'm a big lad and aware I won't be backed up if I manage to injure myself in the process.
As I said before - it's obviously not a gaping lack of thought or anything else - I've been a train dispatcher and I am a guard - including on trains that carry a lot of luggage and have next to nowhere to put it - but I do realise it is a damn near impossible job to please everyone as we are expected to do.
I should of course make clear that I'm referring to the wheelchair space being full of suitcases, catering supplies, pushchairs etc - if it's full of warm and irritated human bodies with nowhere else to stand it does make it rather difficult. Even then though, particularly if the passenger has reserved and depending on the location, I'd probably hang around long enough to get control to book them a taxi and confirm it has been done for them.