While you are outside the UK, you must apply for a Business Visitor Visa. You’ll need to fill out an application online and pay a charge. You’ll also need a valid travel document, supporting papers, and biometric information, such as your fingerprints and a digital photo, if relevant.
The earliest you can apply is three months before your trip, and you should receive a decision within three weeks – though you should check the guide processing dates to see how long obtaining a visa in the nation you’re applying from might take.