“In a couple of years time without Mode S you won't be able even to leave these shores for a spot of European Touring.”
That has not yet been decided, and appears unlikely.
IF I have to upgrade I will probably go for the TRIG. All the reports I have come across are good and it is a plug compatible for my BK. Whatever you do follow the CAA advice and avoid the Garmin 328 or you run a big risk of having to scrap it when phase 3 comes in.
Rod1