BB spaces on private land don’t carry the same legal weight I agree, but under the Equality Act 2010 private service providers have a legal duty to make reasonable adjustments for disabled people, which includes providing and maintaining a certain number of disabled bays. While the official Blue Badge rules don't strictly apply, car parks enforce them to meet that legal requirement.
In most cases I’ve gone straight to the retailer, or where they don’t own the car park, the landowner - it’s pointless approaching the parking company, who will only tell you that you’ve breached their terms and have to pay the fine.
Denying access to goods and services because of disability is an offence under the Equality Act 2010, so if the lack of available disabled bays means that you are unable to access these services without breaching the terms and conditions, you can argue that the retailer/landowner failed to make reasonable adjustments. It’s my experience that if you explain this, most will cancel the ticket to avoid potential discrimination claims.