I'd be asking them exactly whose authority at ATOS is deemed to overrule your general practitioner, and then demand to:
a) speak with that person, and
b) demand they perssnally indemnify you for any health issues that arise from their forcing you to attend.
They won't back down, or are at least unlikely to,
but you can then, after the event demand copies of any records they hold on you under the FoI Act,
et voila, you have some semblance of evidence of unreasonable behaviour on their part that you can present to...well, the media, a tribunal if you're forced to go that route, etc.
Of course, this is predicated on you remaining calm, collected and humble at all times, but most of us can manage that if it means a bit of
schadenfreude further up the road.