Well... there are the technical answers and the feel good answers.
As you have seen, after a hard run the pressure will be lower. The grade of oil you use and engine temperature all affect this as does your idle speed. I believe the hotrod guys (not Triumph guys... just regular gearheads) used to say minimum 10 PSI at idle and 10 PSI for each 1000 RPM up from there until your relief valve opens. The Spitfire/GT6 oil pressure warning switch doesn't come on until about 5 PSI so if you see it... you really have problems. Personally, I would like to see 30 PSI at idle but on hot summer days I am sure our car will hover closer to 25 PSI with a 1000 RPM idle and 20W50 oil. You can improve that by having an efficient cooling system and perhaps adding electric fans.
I am sure others will have their own preferences and observations on normal oil pressure. I have never seen any factory limits for these engines. Hopefully someone will post those numbers if such values exist.