They really weren't less effective.
It was a simple admin deadline - licences have to be renewed periodically, and the NHS decided that it would simply cease administering the single jab at the next expiration point, and without NHS as a customer, the companies decided not to renew the UK licence (even though they had all the required data and it continued to be licenced in other countries, including elsewhere in EU)
Those who have had single vaccines do not require further immunisation.
It was fiendishly bad timing that the Wakefield scandal came right on top of the licence expiration - because of course back then, the question marks had not been dealt with by further research. So there is a cohort of people, roughly in their mid-20s now, with extremely low vaccination rates. They will be at risk themselves, and babies born to unvaccinated mothers will not have any transferred maternal protection.
As an aside, there never was a golden age of high MMR take up. The figures before the NHS ceased providing the single jab were for all those immunised, ie both single and MMR. If the aim was to have high measles vaccination rates, then especially with the controversy beginning, it would have made considerably more sense to go for one more renewal (yes, paperwork needed, but all the required data existed) and let it lapse after all the further studies (which were then only just beginning) produced results.