Hi James, If your in the south of U.K. you will need protection during the winter months they don't like the cold, or frost will kill them. If you plan to plant them out they will need a sunny spot but sheltered from the wind. If it's a #echiumpininana it may grow about 2M but some other grow 1.5 depending on climate. Hope the info help, when potting on they don't like having they roots disturbed, be careful when either potting on or planting out great plants an good luck.
Do you start then from seeds? Expensive plant to be having just for one season they take two to three years before they come into flower! I have grown them in 5l plants and just use a fleece bag over then during the winter. You could try the #echiumcandicans little more stronger and doesn't die back once flower and grow more like a shrub great plant. I have had one go below - ° for a few days and come back stronger.