Temperature wise, Agave americana do fine in the UK. Rain wise, they do badly. So you need well draining soil (70-90% grit to compost). It is worth putting in a sheltered spot (under trees, greenhouse, carport) over winter, the key is to keep the soil dry. When large-at 30 years they are 8ft by 8ft-then they are hardy to keep outside over winter with well draining soil all around the Agave.