From Maurice-Jones, dealing with the consequences of the fall of France:-
"It was decided to surround Great Britain from the Orkneys to the Outer Hebrides with a ring of coast defence batteries which would cover every probable and possible landing place, be it port, harbour, bay, cove inlet, or open beach, very much as had been attempted in the days of Napolean. This of course entailed a tremendous expansion of coast-artillery, an expansion beyond even the wildest dreams of anyone before the war. The first necessity and most formidable problem was the finding of guns. There was scarcely any reserve of coast-defence armament held by the Ordnance Department, but luckily and wisely the Royal Navy had kept in store a great assortment of guns and mountings from the ships that had been scrapped after the First War. From this providential supply, 6 inch, 5.5 inch, 4.75 inch, and 4 inch guns were issued in great number for what were now named "Emergency Coast Batteries" and which were to protect minor ports and cover every threatened beach of Great Britain. Ammunition for these guns was also produced from naval sources, but quantites were very meagre, averaging only about 50 rounds per gun."
He then deals with the other probelms: fire control equipment, and officers and men, noting that initially the latter were provided by RN and RM but GHQ Home Forces sorted out army manning fairly quickly, including using men of heavy and medium regts evacuated from Dunkirk. Installation of the emergency batteries was in 7 instalments Jun 40 - Jan 41 totalling 153 batteries. 6 inch guns were used from Littlehampton to Lossiemouth (138 guns). Most batteries were 2 guns, although some in Kent had 3 or 4 (3 guns needed 4 offrs & 135 ORs) one or two minor ports had two btys, eg 6" and 4". Batteries including 6" were also sent to Iceland and Faroes. Emergency btys were grouped in regts under command of the Commander Corps Coastal Artillery in each Corps HQ with a coastline.
The strength of these batteries started reducing in Jan 43, notably beach defence batteries were handed over to Home Guard or placed in 'care and maint'. There's nothing to suggest that, apart from Iceland and Faroes, any of the ex RN guns were used overseas.
To put the 6"numbers in perspective, on 3 Sep 39 the defended ports had 89 6" and 32 9.2" in UK and 94 & 28 overseas (excluding Australia, S Africa, etc)