The British royals tend to announce and marry in the one year so either the engagement and wedding this year or both next year.
I don't see them really liking the idea of having millions of people waiting in the streets for hours, and some even overnight, during late autumn or early winter any more so I don't see a wedding late this year. That leaves a June engagement and a September/October wedding.
I don't see any possibility of an engagement before the election for the simple reasons that it would overshadow the election, as I have said before, and that is a non-starter. Could you imagine a candidate or two actually promising to cut the costs of the wedding and the consequent one-upmanship that could follow till someone suggests have the royals pay for everything themselves to stop using taxpayers money for anything? These sorts of things happen in election campaigns so it is easier to not have the royals in the spotlight at all during the election, hence why I believe no engagement until after the election.