It looks like a glasshouse whitefly, which are most common in the UK. There are some instances of the tobacco whitefly in the UK, but they're more rare.

Not sure if you're an organic grower or if your apt to use pesticdes. If you want to go organic with killing it, then parasitic wasps are your answer. Insecticidal soap is also great, I've used it before. I've heard that there are sticky traps for them, as well. Whiteflies also exude honedew, which attracts ants, as well as the honeydew can cause mold issues.

So... looks like you have your work cut out for you. :P Good luck!