That's very interesting and comes under the category of scaremongering as Hammond isn't the Defence Secretary now, as the article states, but is the Foreign Secretary. If they cannot get that right I have no faith in the accuracy of the rest of it. Besides which Scottish regiments have long been stationed in England. For example the Argyll and Sutherland Highlanders have a long association with Plymouth and were stationed there in the 1960's.
I see no anomalies here, to me it's fairly obvious.
Anyone sworn to the colours should have the option of deciding which colours they will remain sworn to, not forgetting that there are many English, and other nationalities, in the Scottish regiments, and vice versa.
Similarly with my service pension. I earned my pension in service to the British Crown, and I would expect the British Crown to continue paying it, just as does for all the other foreign nationals who have qualified for a service pension.