Most people have heard the "every six months" rule, but the honest answer is that the right interval varies from person to person. A young adult with no fillings, strong enamel and good brushing habits may only need a yearly check-up, while someone with gum disease, a history of decay or a dry mouth is often better off going every three to four months.
A routine visit is not just about finding holes. The dentist checks the gums for early inflammation, looks for worn enamel and bite problems, screens for oral cancer and, where needed, takes a small x-ray to catch decay hiding between the teeth. Catching a problem early usually means a cheaper, simpler fix.
If any of those show up, do not wait for your next scheduled appointment. In London the mix of private and NHS practices means you can usually find a same-day slot somewhere, though prices and wait times differ a lot between clinics, so it pays to call a couple before you book.