Only as an idea I looked at Mustang 3.7 V6. It is a US site, but even with shipping the price would be good in comparison. I don't think that Morgan uses any other sensors than Ford does?
Even in UK Ford Mustang sensors should be cheaper than "Morgan" sensors.
BTW I have always had good experience with NTK.
https://www.rockauto.com/en/catalog/ford,2015,mustang,3.7l+v6,3307632,exhaust+&+emission,oxygen+(o2)+sensor,5132
