Most Western and Gulf passport holders do not need a visa for a short business trip to the UK, but visa-exempt visitors (including US, EU, Canadian and Australian nationals) must hold a UK Electronic Travel Authorisation (about GBP 16 / USD 20, valid 2 years, multiple entry). Nationals of visa-required countries need a Standard Visitor Visa (about GBP 127 / USD 160, typically decided in around 3 weeks), which permits attending trade fairs, meetings and negotiations but not selling goods or services directly. Apply well before booking non-refundable travel and carry a letter confirming your trade-show attendance.
Estimated cost: Varies by visa type and nationality (check official site).
Processing: Varies by nationality; apply several weeks before travel.
Apply online at gov.uk before travel and give biometrics at a visa application centre. Permits attending trade fairs, meetings and negotiations but not selling goods or services directly to the public. Valid for 2 years, multiple entries, stays of up to 6 months. Applied for online through the UK ETA app or gov.uk.
Official