The LibDems are wrong to say that MMR is the only safe option. There is nothing wrong with the single measles jab, and its licence lapsed for administrative reasons, not because of concerns over safety or efficacy.
It is the preferred option, and the only one available on NHS. If parents have £400 to spend on single vaccines rather than using the free MMR jab, then that is their choice. The important thing is that children receive a vaccine with a measles component. If parents won't give them MMR, then it is clearly better, for both individual child and the herd, that they are vaccinated by a method parents find acceptable.
I don't think that parents are right to shun MMR. But if they are set on that, I'd rather see children vaccinated singly than being unvaccinated.