As noted, DCB Legal aren't the types who tend to follow through to a hearing, so a lack of response to a Letter of Claim is unlikely to affect the process in the slightest.
Speaking more broadly about court claims in general... It's generally advisable to respond to Letters of Claim, and indeed as the PAP says, courts expect parties to have taken steps to resolve the dispute before proceedings. If one has already done this in the form of, for example, an appeal to the parking operator, POPLA or the IAS, then it could be argued that one has already sought to resolve the dispute, but there'd be little harm in reiterating the argument in response to the LoC. If one had not appealed at all, and then ignored a Letter of Claim, if it did go through to a hearing, a judge may take a dim view of this, but even then, it ought not to swing the outcome of the case overall.