To do what?
Off the shelf is difficult although EPP is standard, generally different registries tend to have different requirements so you generally need a slightly different build from registry to registry to cater for legacy stuff.
WHMCS (and most other billing systems) will handle all the day to day stuff if you want an off the shelf system rather than just a client, I built my own code library to handle my requirements, there's plenty of sample EPP code out there, and Nominet's schemas are well documented.
Unless you have a lot of domains, then I'd suggest using Nominet's web based domain management as it's free and easy to use.