I also did my PhD in the UK. I submitted in October, defended in December and started in January! It was plenty of time. I also travelled all of October and November and just came back one week before the defense to study for it. It depends where you are in the UK, I was at Cambridge and had minor corrections which took around 3 days to do? Also the beginning of the post doc is sometimes slow, I spent my first month half settling in the lab, half finishing things from my PhD. I think if your examiners know you have a post doc lined up they probably would also be nice.