There is nothing to appeal, the police have just sent you a notice requiring you to identify the driver. You've been notified, so to that extent the notice has served its purpose. There is also a request for driver details under section 172 of the Road Traffic Act 1988, and you have no choice but to fill in the notice and send it back: if you don't you're looking at 6 points and a large fine, and failing to send the notice duly filled in is a much more serious offence than doing 25 mph in a 20 zone.
Once you send the notice back, the police will decide what disposal to offer. If you were the driver then once you hear back from the police you can decide whether to accept any out of court disposal (such as a driver improvement course, that would avoid any points) or go to court and plead not guilty.
If you were the driver and want to go to trial, you'd have to tell us what defence you think you might have, for example if there were no signs saying it's a 20 zone.
I'm guessing the location is
here but as the Google Street View images are several years old, if you want to dispute the signage you would need to go back and get photos.
