Then even if you were not initially in contravention, by the time the CEO served the PCN you were. In the circumstances, your best bet is to make a plea for discretion, which for a blue badge holder will normally be accepted more often than not.
Here's a draft:
Dear London Borough of Camden,
I would like to explain that at the time of the alleged contravention I had been accompanying my disabled brother (who is a blue badge holder) across the road into Camden Market for a meal. Unfortunately I forgot to display the blue badge on this occasion, I should add that due to his medical condition I had to assist him from the car and this would have somewhat distracted me.
While I understand that under the law this is no excuse, I would request that on this occasion you exercise discretion not to pursue the matter further, I will of course be mindful to ensure full compliance going forwards.
I am uploading a copy of my brother's blue badge for your records.
Yours faithfully,
Make sure to keep a screenshot of the confirmation page.
He has appealed this, thank you for your help.