Given a travel related situation, and given a specific question about the situation, summarize the question into a title
Q: How soon can I return to the US from the UK on an ESTA after abandoning the K1 visa

Background

My finacee and I were going to marry in the US as she was doing a PHD there (she is American). However whilst I was over there on my K1 visa, she decided to transfer her PHD to the UK. So we didn't go ahead with a wedding there.
I returned to the UK after spending the maximum 90 days there. Now I would like to return on an ESTA for a month or so. Ideally, I want to go back right away, I know much of the decision will be made at the border, but I can explain the situation well enough and i'm not concerned with the border officer not believing me as I have the expired K1 with me. I'm just wondering if I may have missed anything obvious? And if I get denied at the border, what impact it will have on future ESTA travel.

A:
How soon can I return to the US from the UK on an ESTA after abandoning the K1 visa?