If this is what they offer it really is going to be up to you to demonstrate why mid wheel drive is unsuitable.
Thats easy. Get close to them, and turn. The rear casters sticking out will rip up their feet. Already done that. Tell them that this isnt safe in the pub, you will be killed... Tell em 55Ah batts will last you half a day. And you might want to go out in the evening to eat. Then go outside on your lawn, reverse over an uneven bit till you get stuck (not hard) and let them try to get you unstuck and rescue you. Explain that one of your interests is photography and model planes and both involve grass or awquard to access places. If they say thats no interest to them. Explain what a HOLISTIC approach, and JOINED UP THINKING actually means. Because thats what it says on their own NHS website! Ask them if they have any clue what a powerchair assesement actually is?
And the stupid little casters set off my muscle spasm making the thing a painful nightmare on what passes for our streets. Shopping centre specials. That might noyt apply to you.
Your self assessement needed to have already explained all of this in detail. Leaving them nowhere to go. If this is what you need. YOU must explain why. Then go over the chair they want to offer you in fine detail explaining point by point why it isnt any good.
They will make a lot of notes and run away as fast as poss as you will make them feel like they are out of their depth. They will offer you some unsuitable chair by post.
If not ASK FOR ALL THE DETAILS. And then see what it is. And assess it, return your assessement of their chair. Unless its what you need.