Because you are in the UK, you might check some online Honda forums for other vehicles that have similar model year NAV units that are available in Europe. The units in the U.S. may be different. This is not a popular mod around here.
I did a search and found several threads that seem to go nowhere. Here is one that has a little information:
I think it will be important for you to figure out what NAV unit you have (for instance, in 2012 the units changed here in the U.S.) You may even have to take it out just to see if there is a camera input. There is a thread about removing the audio units. You may be able to look at the back of the unit by removing (or just dropping) the glove box and using a small mirror & light.