Semi hysterical responses to a low key problem adds nothing to solving the problem. A more reasoned response might well have been to politely ask the security guard to address the wheelchair user directly and/or use some of the logistical solutions identified earlier on in the thread. A calm polite letter sent to Boots suggesting disability awareness training would be as far as I would go if, and only if, I hadn't challenged the security guard over his behaviour.
From the OP:
Again, the security guard was addressing me so I asked him to address S instead, as they were the one with the wheelchair. The security guard refused, stating (and this is an exact quote, bar changing the gender) "Yeah but <they> doesn't want to listen".
Sounds like he was challenged, and refused to alter his behaviour, hence the advice to write to Boots about it.
Any of us should think about how we'd like to be treated in that situation. When NT ruptured his Achilles last year, I took him out in a wheelchair a couple of times, just down to the supermarket for a change of scene, when it was too far for him to manage on crutches. Several times people patronised him (other customers, I should stress, not staff), one old lady, a complete stranger said "ohh, you silly, what have you done to yourself?" in the sort of voice people use to talk to babies.
I'd want people to address me when it was specifically relevant, and otherwise to include me just as they would include more than one person in a group all standing. I wouldn't much care if they leaned or crouched, as long as they didn't treat me like an idiot!