There is no registration for a small farm or smallholding. Your CPH number *is* the registration for the holding, what other registrations you need to do depends on what you intend to use your land for.
If you're going to keep livestock you'll need to register for the relevant species, each of which have their own registrations -
herd number for cattle,
flock number for sheep, and so on. See the relevant species guides on TAS for more info on each -
http://www.accidentalsmallholder.net/livestock/If you're going to be claiming grants or subsidies you'll need to register with the relevant agency - Rural Payments Agency in England and Wales, SGRPID in Scotland.
Hope this helps, if you need more info please share where you are and what you're planning to do.