OP, parking on the footway in the Greater London area is a statutory prohibition which therefore does not need to be signed. Whether you knew this or not is not going to avail you should this get to adjudication.
https://www.legislation.gov.uk/ukla/1974/24/section/15Similarly, IMO the 'unloading' exemption isn't available to you because you weren't, you were working away from an unattended vehicle.
As regards 'I have not received a response from Brent in reply to my representation', I notice that you didn't include your address in your email reps(perhaps you did by other means, I don't know). You've also not posted the rear of the PCN, so it's possible that your reps were disregarded because they were not formatted as required.
NTO posted 27 Oct. therefore the addressee has until 25 Nov. to make reps. You might as well because the discount isn't on offer at present.
Were you parked
wholly on the lowered footway, that is the vehicle crossover to the property where you were working, or did part of your vehicle stray into the crossover for next door? You might have more compelling reps if you were wholly on your client's crossover. I cannot tell from the photos, but as you were there you could tell us.