In recent years, some of the Westland Lynx helicopters of the Portuguese Navy have undergone some form of mid-life update (MLU) in order to have their engines and avionics upgraded. Also, allegedly, a few Portuguese naval aviators have undergone training and qualification on these updated Lynx helicopters. Allegedly, such work took place at Leonardo's site in Yeovil, Somerset, England, United Kingdom.
Interestingly, articles on the aforementioned events do display photographs that show a British roundel alongside a Portuguese roundel on Lynx helicopters of the Portuguese Navy. The following photograph shall corroborate my claim.
Why would one paint a British roundel on a non-British Lynx?
At the moment, only one hypothesis occurs to me — though the aircraft being upgraded do belong to the Portuguese Navy, the test pilots are British.
On Portuguese Lynx helicopters with British roundels:
Marinha Portuguesa, Helicóptero da Marinha Portuguesa realiza primeiro voo após processo de modernização, 18 de fevereiro de 2020. [Google's Portuguese-to-English translation]
On Brazilian Lynx helicopters with British roundels: