RoRo implies a specific type of transport. For the UK, RoRo relates to any kind of transport from the UK to the mainland EU (or the other way round), which is either done by ferry (east coast, Dover,…) or via the Euro Tunnel (by train). Furthermore, RoRo may relate to goods transported on trucks (with a driver) or unaccompanied trailers (without a driver). The customs clearance location is most likely to be located near a port.
In the UK, not all customs clearance locations can be used for the RoRo use case; only the ones listed on the following page: