File an online report to Merseyside Police at https://www.merseyside.police.uk/contact/af/contact-us/us/a-matter-involving-driving-vehicles-or-a-road-traffic-incident/
You’ll be given a reference crime reference number at the end which you can use to follow up if needed.
It’s very useful to have video footage, you will be sent a link for uploading it. Two minutes either side of the incident is needed as well. Do not share footage on social media as this could prejudice a case.

An example of the type of wording to use in your report:
Please tell us as much as you can about the incident, and the events leading up to it, to help us identify if an offence has been committed:   
At approximately 14:27  hours on 17/05/2020 I had come off the flatter part onto the hill – this is where the speed limit changes from 70 to 30mph. Driver of the [car type] passed me far too close and at speed – made no attempt to slow. Oncoming cars in opposite lane made it impossible to give enough space. Driving like this is very intimidating,  dangerous,  aggressive.
My chest mounted camera, a  [put make & model of gopro/next base whatever your device is] which records images from the front and rear.
The above incident which I have described was recorded on my [whatever you device is eg. Nextbase go pro etc.]
I have transferred a copy of this video to Merseyside police via the internet and a link that was provided by Merseyside Police.
I have not knowingly altered this video footage before doing so, but have simply uploaded a section of it as requested. I exhibit the footage as [your initials + 1 eg. EL1]
The date and time displayed on the video is correct starting at approximately 14:27 hours on 17/05/2020 – (put the actual time & date that is shown on your device if it is not the same time & date of the incident).
I am willing to attend court and will abide by any decision made by Merseyside Police.

Article on Road CC about reporting with camera footage HERE

Apparently the Nextbase system is not working properly and it’s best to use the Merseyside Police system above (May 2020)
Merseyside is now part of the national system Nextbase.
https://www.nextbase.com/en-gb/national-dash-cam-safety-portal/
The system is used be Merseyside Police to monitor and process incidents and is a much more streamlined platform for viewing and acting on reports.
MCC member Ed Lamb has created a video to show how to use the Nextbase system. https://youtu.be/AY_cqrKbViM

Collideoscope
Collideoscope invites you to report cycling collisions and near misses in the UK. It collects the reports together and makes the data available to planners, researchers and campaigners with the aim of making our roads safer for all. Collideoscope is a joint project from mySociety and the Merseyside Road Safety Partnership.
You can report actual collisions between a bicycle and another vehicle, or near misses where an incident was averted. Additionally you may report incidents where no other vehicle was involved, such as collisions or near misses with pedestrians, street furniture… or anything else.
Do not report incidents that have been reported to the police and are under investigation as this might prejudice any resulting court case.
www.collideoscope.org.uk