I thought I wasn't worried about London, until I woke up a couple of nights ago really stressed about it. I think running somewhere familiar in a much bigger race might settle me down. I think too that I will be so excited about being in London for the first time, that it won't bother me overly much that it's not my first 10k.
I think the answer is, just do it!!! It will definitely help me prepare, and give me a bit more confidence heading into London. And if I can't make it, I can pop into work, which is on the route, and have a cup of coffee and a lie down