I suspect that, rather like the RJ and F-35, the RAF colour scheme for the P-8 will reflect the US one with only minimal changes to reflect the UKs ownership. When every penny of the procurement process is scrutinised intensly the cost of changing the paint scheme significantly is simply not worth it.
As regards number plates the rules do not seem hard and fast, whether they should be is debatable. History and tradition are important and the RAF should cling to what it has. Genuine question: why is 120 (or CXX if you prefer) a special case like 617?
I'm surprised the thread hasn't asked the question as to whether we should keep the Poseidon name or change it to something else yet!
