There is nothing to defend at this stage. You must first name the driver. The police will then (most likely) will offer of a Conditional Offer of a fixed Penalty (£100 and 3 points), which you can either accept or ignore. Ignoring it will then see it move to court phase via a Single Justice Procedure Notice, this may not happen for several months. Only when you get to the SJPN stage is any evidence the police going to rely on required to be released.
Ignoring the CoFP could be an expensive mistake, the police will have apple evidence of the offence and the vehicle in question. And since you will have named the driver it is pretty much a slam dunk for them.