I think that this is correct. I have been told that when buying on ba.com the ticket is, by default, sold in the country in which travel originates. So when I buy a South African domestic on ba.com, it's priced in ZAR and the ticket is issued in ZA. (And therefore, occasionally, shuts down my credit card because the ZA transaction trips a fraud detector.)

You may be able to check whether UK availability is indeed different by selecting a UK point of sale on ExpertFlyer.

Note that you can't MFU either an N class or S class ticket, so your reason 2 shouldn't affect your decision.