the worst thing you can do is keep looking back. the way i see it, you have two choices. carry on or turn off.
1. carry on... to do this you need to just basically get into the habit of ignoring the lorry. they are bound to pull in close before overtaking, to make their overtaking distance shorter. you may not like it, but if you're gonna choose this course of action, you just have to trust that as he's seen you, he is well capable of handling his lorry, if he's tailgating you, although it seems scary, he is only doing your speed (say 15mph?) and lorries do actually have amazing brakes, and by constantly looking back at him you are more likely to crash yourself, causing the pile up, and thus end up under his wheel because you weren't looking where you were going. so stop looking back and concentrate on your own job of riding your bike.
2. turn off. to do this doesn't mean you need to suddenly slow down at all. look ahead for a convenient spot. when you see it. hold you right hand out behind, palm towards the driver, while taking a quick glance behind at the driver (thus giving him an indication to back off). then, turn your hand up into a thumbs up position (to thank him in advance) and return your right hand to the handlebars, and then immediately indicate that you are turning left. the lorry driver will use his intellect and know you are going to slow down and allow him to pass. the other signal you could do is the highway code signal for slowing down (can never remember it myself but i'm sure it's a circular motion with your right arm).
that's how i would handle it.
personally, i find HGV drivers the most considerate when overtaking, but i don't live in London, so i could have a distorted view. bus drivers however... now they are nutters! (except for Johnsons coaches who i actually wrote to once to thank them for their consideration!)