Yes, there is
case law on the Nominet website.
Had a quick skim through some of them for you and it looks like there are a couple that mention the DRS process, which is what you want to know about by the sounds of things.
Firstly, in the Citigroup v GPM Ltd case, it is stated that the dispute had originally gone to Nominet's DRS but had to be
"suspended when it went to court". In my view, this would show that the two are independent of each other and don't need to take place subsequent to each other.
Secondly, in the Tesco v Elogicom Ltd case, Elogicom raised the defence fact that
"Tesco should have used the DRS and UDRP rather than going to court". In response to this,
"the judge said that this was not compulsory, and both the DRS and UDRP made it clear that they were alternatives to court, not mandatory and neither could order costs and other similar remedies".