I was advised to try and ask for further evidence in relation to the captured images and if there is video proof?
There is unlikely to be any. There certainly won't be any video evidence.
and if I am found guilty to ask for mitigation?
Mitigation relates to factors which seek to reduce the court's penalty by lessening the seriousness of the offence or claiming your personal circumstances warrant a reduced penalty. You don't ask for mitigation, you present it for the court's consideration.
Who is giving you this advice?
You can change your plea at any time up until the prosecution closes its case in court. The first thing you will be asked in court after confirming your identity is whether you want to maintain your guilty plea. Alternatively you can try contacting the court before the 5th to inform them.
A guilty plea at the earliest opportunity attracts a discount of one third off the fine. "At the door of the court" will see that reduced to, perhaps 10%. Similarly an early guilty plea will see the prosecution ask for just £90-ish in costs. The amount they ask for at a late stage is up to them, but should be less than the £650 I mentioned if the trial went the distance.
