Nothing you did was unreasonable
It's the clients that think it is our responsibility to chase them up and don't take any responsibility for their own affairs that are unreasonable. They have 10 months to get their stuff together and I presume you'd reminded some time earlier in the year.
Frankly he was blinking lucky you filed on time, after all he also probably presumes that you were sitting round with nothing to do just awaiting his call...
The question is how much extra you charged him, was it more or less than the £100 penalty?