No. United Kingdom doesn't ask visitors to fill in an online arrival form (also called an entry form, immigration card or landing card) before they travel. No arrival form. Some nationalities need an electronic travel authorisation (ETA, GBP 20).
Do I still need a visa for United Kingdom?
An arrival form is not a visa. Whether you need a visa or an electronic travel authorization depends on your nationality, so check United Kingdom's official immigration website or your own government's travel advice.