OK. Firstly to register the operator and drone the site is here:-
Overview of how to register and take the test to fly drones or model aircraft legally in the UK.
register-drones.caa.co.uk
Depending on drone you may need to register both or just one. Depends on the weight.
You'll definitely need an operator ID and might or might not need a Flyer ID depending on the drone.
Its the same site and system for a tourist and all done through there.
Online, quite painless and even with the test takes about 15 mins max.
Then make sure you're familiar with the UK drone laws and also where you can/cant fly.
Absolutely no not EVER rely on DJIs inbuilt Geo to see where its legal to fly - its woefully inaccurate and misses out most areas.
Also be very cautious of 3rd party apps for the same reason.
Get and use the official National Air Traffic Services "DroneAssist" app. This is the only proper authoritative data source for where you can and cannot fly so should be the one relied on.
Altitude Angel (who provide the backend) also have a website where the same information can be seen:-
Registering a drone or model aircraft | UK Civil Aviation Authority.
Definitely don't rely on other software to see where to fly.
Some places like National Trust <spit> and others wont allow you to *operate* from their property (ie take off, land or control from). Some like NT also falsely claim its illegal to fly OVER their land. Its not. Provided the rest of the rules can be followed to regarding VLOS etc its perfectly legal.