You book the theory test through one official government service. Anywhere charging more is an unofficial middleman — avoid them.
You need a valid UK provisional driving licence. You can apply for one from GOV.UK once you're 15 years and 9 months old (it costs £34 online). You can't book a theory test without it.
You can move or cancel your test through the same service. To avoid losing your fee you generally need to give at least 3 clear working days' notice. Keep your booking reference handy.
The car theory test is £23 — full stop. Unofficial sites add a booking fee for the same test. Only ever use the official GOV.UK service.
Book once you're consistently scoring 45+ on mock tests and passing hazard perception practice — a re-sit costs another £23 and more waiting. Not sure where your nearest centre is? Use our test centre finder.