Potential client sends pics that concern me

[M-B-Electrical-Services]

If it was me, just by having those pictures being sent. I would have told the client straight off the bat that is a C1 and if the cover was not on site to be found, a new CU would need to be installed. See what kind of response you got before making a decision.
Not being critical OP but if every electrician was to just knock back these kind of jobs, they will never get done other than by Dave down the pub.
Send him my details

Hi Dave.

No offence taken at all mate. Its a valid point you make. I made my decision based on my current workload and ability to get the job booked in within a reasonable timescale and I just couldn't make it work.
What I did do though is send a detailed email explaining my concerns regarding the obvious C1 being shown and what would be needed to rectify it.
I clearly explained the urgency of the situation and apologised for my inability to attend the job in the timeframe that the job required.

The lady thanked me for the information and honesty about my lack of availability.

I feel I handled the situation as well as I could. I would be happy to do the job as I've taken on much much worse but I simply had no availability when I looked at my diary.