PoFA could be one angle as above.
- The driver is 70 years old, has English as a second language and has multiple health conditions
Of your proposed grounds, I'm not sure this is particularly compelling. It's unclear how being 70, or having multiple (so far unspecified) health conditions affects his ability to read the signage. MET could seek to counter such arguments by saying they rely on the assumption that drivers who use car parks they operate in are fit and well enough to drive, and therefore ought to be able to read and understand signage they encounter in the course of said driving/parking.
That the requirement to register was less prominent may have more legs, although it's definitely not the worst signage I've seen.
Meanwhile, go back to the sports centre
Speaking to the council about the actions of their supplier's subcontractor might also be worth a go.