That's the one lead I can offer - might want to try contacting that guy although it seems he doesn't know much more than what he alludes to there.
(referring to you )
I'm fairly certain the only photo of an NSX in DA
(Driving Ambition the godly book for all things McLaren F1 road car related) appears on page 25 and is a press photo of a yellow one, so not this car.
Another thing worth mentioning is that the car shows registration to "McLaren International" and not "McLaren Cars Limited" so it may have had nothing to do with the F1 road car development program. In 1991 their Formula One cars were still powered by Honda and Senna was one of their drivers so who knows - this car could have been driven by the master himself? Reference these articles by Andrew Frankel - could have definitely been this car.
A Google search for 'Senna NSX' produces quite a few images of him with a red car but no shots of that UK plate referenced in the logbook unfortunately. There is one NSX that has been for sale in Portugal but that's most likely a different car.