The agreement to have fixed term parliaments? I think an election will happen if she is given an excuse and it will make 1983 look like the proverbial vicarage tea party.
There's the fixed-term parliament thing, but there are ways round that. More important, I think, is the desire to keep the left of the Labour Party in control as long as possible in order to maximise the damage and ensure that there's no worthwhile opposition to make things difficult during the Brexit process. A shattering electoral defeat would force Corbyn out and precipitate a leadership election that a moderate would win. That's the last thing May wants right now.
Ah - I thought that was just an agreement when the coalition was in place. But yes - it does look that way regarding the outcome.
RE Ukip - the fact that when looking for a leader after Farage, Nuttall, the deputy, wasn't considered until after another leader had come in and resigned and the candidates to replace her completely screwed themselves just shows how little credibility he has. To survive, they needed some prominent defection from the Tories I would say - and if they thought Carswell was in any way that person they deserve to disappear