How old do you have to be to be added to the insurance if you work at a car dealership (UK)?

You'd need to ask the dealer. There is no one size fits all rule here.

To demonstrate a vehicle to a customer or deliver a vehicle over the road, it is age 21. To move vehicles around the dealer lot for service / cleaning etc, it is 18.

It's 18 in the USA.

AGE is NOT a factor. If you're LEGALLY EMPLOYED by a car dealership and in a position to drive customer's cars, you're covered under the company's liability insurance.

That is something between the dealership and their Insurers. Whatever your manager says is what the rules are.

Some of the requirements are dealer specific. That requires you to ask the Dealer(s) what their requirements are

Many dealers do not hire anyone under 25 for insurance reasons. They COULD hire any age but it might not be worth the insurance prices bump for the dealer for anyone under 21 for sure.

The dealership will tell you what the parameters are. There will be stipulations like age and full, clean licence.

motor traders insurance ..it covers all employees ..you would need to be a qualified driver ..and therefore old enough to drive the intended vehicles ..18 should be ok ..though some companies may put restrictions on certain vehicles

Varies from company to cpmpany. Where I work, you can drive some of the vehicles when you are 18, but the more powerful models are over 21. But you have to sit and pass like an advanced driving course.

So most dealerships that are not owned by the manufacturer will have a kind of "blanket" cover as their staff will drive many many vehicles in the course of a day.

Talk to you manager and she/he will be able to explain the detail.