The questions (2) are
(1) From what I understand, UK VMC minima when above 3000 AMSL but below FL100 are 5km visibility; 1500m and 1000 feet from cloud. However, I cannot visualise how this works.
First of all its AT or above, not above;
Try and understand why the legislation exists, it’s to protect aircraft from flying into the scenery and each other. Airspace at or above 3000 feet gets a bit more serious with more en-route public transport flights and faster civilian aircraft that need more time and space to manoeuvre. What the legislation seeks to do is to put a bubble around an aircraft to provide this time and space, whereas you are unlikely to reverse into a mountain another faster aircraft can approach in any direction or/and could be climbing or descending too. The mix of units is purely because we measure visibility in metric units and vertical distance and visibility in feet in the UK.
(2) Secondly, can PPLs without any additional ratings fly out of sight of the surface?
Answer, no, never-see schedule 8 of the ANO.
FLIGHT CREW OF AIRCRAFT—LICENCES, RATINGS, QUALIFICATIONS AND MAINTENANCE OF LICENCE PRIVILEGES.
I wont do a Beagle and demonstrate copy and paste ability or ability to over complicate and confuse, you should seek it out and read it yourself, that’s one of the aims of the exam, familiarisation with the definitive, legislative documents
You are confused because you are dealing with two separate bits of legislation, one on VMC minima and one on Licence Privileges. Its good to read books on aviation law buts its also beneficial to refer to the statutory instruments.