Do children over three years old need to be in a car seat in the rear of a vehicle?

Do children over three years old need to be in a car seat in the rear of a vehicle, or can they use the standard seat belt without a car seat? I'm reading the .gov website and one page states children...

My son needs a car that can take three child seats, what are his options?

My son has already has a daughter and is now expecting twins. What cars can fit three child seats side by side, and do these have to be Isofix fittings?

I want to replace my Ford Fiesta with a classy, four-door hatch, what do you think of the A-Class?

I've got a very nice 2018 Fiesta ST Line X with a lot of spec on it. But, because my partner and friends have various back and mobility problems, and I need to transport grandchildren, I'm now looking...

What is the law with children travelling in the back of classic cars without seat belts?

Can you please tell me the law on children travelling in the backseats of a classic convertible without seat belts?