I am a Guyanese and need a visa to attend an international meeting in Spain for 3 days?

What kind of Visa will I need to apply for? There is no Spanish Emby in Guyana and the cost of airfare from Guyana to Venezuela to apply for a visa there through the Spanish Emby in Caracas is prohibitive. I will be ping through the UK en-route to Spain. Can I apply for and secure a Spanish tourist visa in a day or two from the Spanish emby in London and then travel on to Spain to attend the Conference?