Rimmer is right,
You can hold as many heads (expanding or otherwise), cases and primers as you want. Until they are made into a complete round, they don't count towards your "to hold" count.
You must have a valid FAC in order to purchase primers, which was introduced under the Violent Crime Reduction Act 2006, but there is no requirement for them to be listed or at or under the amount of rounds you are allowed to possess.
People often say that you can hold as much powder as you like, this in practical terms in near enough true, but there is an upper limit to how much powder can be stored on a premises without that premises being registered. My memory is a little vague on this point as to exactly how much powder that is but something tells me it is around the 25kg mark, which to recreational reloaders for instance loading a .243 would equate to around 7000 rounds.
As far as I can remember, this is an EU regulation/law which has been in being for a while but I've never heard of anyone being prosecuted for breaching it.
As for RFDs refusing to sell you ammunition until you have agreed to have them entered on your FAC is the product of the same Licensing Departments that create all the fear amongst totally legal firearm owners by asserting power they do not have. There's no requirement for anything to be entered on an FAC unless they are complete, functional rounds. You will need to produce an FAC to purchase expanding heads however as they are authorised by a specific condition which, for instance, a target shooter would not necessarily have.