Please report dangerous drivers to the police - Roadsafe. Report to plod, not just whinge here.

You should report him to the police but I don't think given his behaviour a road safe report is sufficient. You should definitely report his driving, but within this bit:

I then pulled over to write his reg and company name down on my phone, but he sees me and slams on his brakes (stopping traffic) to have a word with him. He is really aggressive, pointing at me and telling me to "watch my fucking mouth". He told me that he took a wide berth to overtake me and that I should have taken a tighter line. He said that I shouldnt be riding a "stupid fucking push bike". I told him its single lane traffic and that youre not supposed to overtake on a left turn like that. He tells me to fuck off etc and to watch my back and drives off.

There seem to be several potential public order offences covered by the Public Order Act, possibly * Section 4A - Intentional harassment, alarm or distress * or Section 5 - Harassment, alarm or distress which cover the threatening behaviour:

*Threatening behaviour' is covered by Section 4 of the Public Order Act, 1986.

"Section 4 - Fear or Provocation of Violence

A person is guilty if he either

a) uses towards another person threatening, abusive or insulting words or behaviour, or
b) distributes to another person any writing or sign which is threatening, abusive or insulting

and either

i) he intends to cause that person to believe that immediate unlawful violence will be used against him or another by any person or to provoke such immediate violence, or
ii) it is likely that the person will believe that such violence will be used against him, or it is likely that such violence will be provoked." *

N.b. I am not legally qualified at all, that's off the internet and should be treated with a pinch of salt but seems to check out. But you should do some quick research and report him for that too because he does indeed sound like a right nutjob.