Just go. Go now. It has a little while to run. You can book tickets and find out more here. We went today. It is a breathtaking exhibition. They've got some stunning things to see, including the actual capsule used by the first woman in space, Valentina Tereshkova. It looks more like a wrecking ball than a spacecraft, covered with insulating material and with a hole where you get in and out.
There are things I wasn't sure were still even around, prototypes of satellites and lunar landers and some completely awesome artwork and drawings. I'm just about old enough to remember some of this when it was happening and I find it amazing that all these years later we can go and see the actual stuff. The only snag (for me) is that you aren't allowed to take pictures. But you'll bring back a whole bunch of great memories.
(and a few postcards and T shirts - which are half price at the moment)