Oh dear. What pity you didn't come here first before making that self incriminating appeal.
In the email, the author clearly identifies themself as the driver by using first-person admissions of driving/parking activity, e.g.:
“I have a valid visitors permit and it was on display.”
“I was parked in a disabled bay and had a valid blue badge showing in the front windscreen.”
Those statements amount to an explicit admission of being the driver on the material dates. As a result, the operator can proceed on the basis of driver liability and arguments about PoFA keeper liability would no longer assist.
For future reference, only ever refer to the driver in the third person.
OPS are a horrible, vexatious firm of ex-clampers that operate in the Brighton area. Had you not identified the driver (no legal obligation on the Keeper to do so), you would have had a very easy win.
Also, you should never respond to a Notice to Driver (NtD) windscreen ticket until the very last day (day 27 after the date of the alleged contravention) as this usually forces them to respond (reject) the appeal without issuing a valid Notice to Keeper (NtK) thus breaching PoFA para 8.
However, you are where you are. You have almost no hope with an IAS appeal but you would almost certainly win if this ever proceeded to a court claim. However, that process will take many months (9-12+) before it concludes.