If Universal want the CP Farm and Elms Farm land the easiest way for them to achieve that is to make an offer to the landowners to purchase that land. National or local government can issue a Compulsory Purchase Order when it is in the public interest to do so, although I think public interest is normally defined in terms of infrastructure development, major housing development, etc, rather than a straightforward commercial development like a theme park. If a CPO is issued the landowner needs to be compensated at market value, which circles back to the point about Universal just buying the land in the first place.
If Universal were hoping that further land acquisition would happen by Compulsory Purchase Order I'd have thought that they would have included it in the Special Development Order planning application to the government? The SDO application is so wide ranging that including longer term plans for expansion by way of CPO land acquisition would have ensured a way forward for Universal, given that is certain that the government is going to approve their planning application. As far as I can tell, there's no hints that Universal is planning on acquiring that CP Farm/Elms Farm land, and there are active proposals for housing and solar farm developments, so I can't see that bit of land being part of Universal's future. Although having said that, so much of the work to date have been kept under wraps it's anyone's guess what else Universal might be planning beyond what has been made public.
I think you raise an interesting point about visitor numbers and park size/attraction count. As I understand it, Epic Universe is currently running well under the attendance that Universal is targeting for the park at Bedford, but despite having more attractions than we've guessed at for USGB's opening roster we are seeing extremely long queues in Epic's first summer season. Have to say, that makes me a little nervous about the kind of experience that guests at USGB are going to have if that park really is hitting 8.5m in the first year of operation.