O... kay. Why so limiting - and why is that not part of the conditions in the first post? The Radical and Caterham are both fully road-legal production cars from 2015 or earlier with naturally aspirated, 1.6-litre (or less) engines, meeting the conditions you set.
Nixxing convertibles means no Mazda MX-5 (8.3s for the 1.5-litre ND), no Lotus Elan M100 (7.8s for the non-turbo 1.6), the third-gen Lotus Elise (6.0s for the 2010 1.6-litre) - and so on.
If you're arbitrarily disqualifying cars that meet your stated conditions, how is anyone supposed to give you a satisfactory answer? Someone may suggest an AWD car but you might want front-wheel drive only, or a Japanese-market car but you want official UK cars only. Give us all of the conditions you need rather than waiting for people to suggest cars you don't think are appropriate.
What's the goal here, aside from winning a discussion you're having with colleagues?