Is it possible to track someone using their mobile number in the UK?

This is something I’ve wondered for a while – is it theoretically possible to track someone using their phone number in the UK? It’s a classic Hollywood trope and I’m wondering just how much truth there is to it and how feasible it would be.

Doing some basic research returns plenty of services and apps that claim to be able to do it. For example, here:

Our location services use a variety of technology to locate a handset. Primarily we use cell-ID to locate a mobile phone to within a certain transmission cell. Accuracy is greater in areas of high population density (e.g. large towns and cities), and reduces as population density reduces (e.g. in the countryside)…

Our service will not work with landline numbers. It will only give the location of mobile phones registered to a UK mobile cell network. The service does not require the user to install any apps or other software. The service works cross-platform, which means it can work on Android, iPhone, Windows Phone, and older basic phones.

…and here:

So if you’re concerned about the location of your family members, want to know the location of your employees, find a lost phone, or want to track down a suspicious call, phone number tracking is as real as they show in the movies!

I also came across this which seems like it could be of interest.

Is it possible to track a phone using a UK mobile number, and if so, is it limited to corporate entities? How easy or hard is it to do for the average technologically capable individual?