The thing is karaokemark, it's very likely that a large-scale attack WILL be carried-out in the UK sometime in the future. When it is, it will most likely be a spectacular event on the scale of, or greater than, that of the 1996 Docklands bomb. The security services in the UK have admitted that it is mainly by luck that they have managed to avert previous attempts.
Having said that, I wouldn't have second thoughts about travelling to London, Istanbul, Ankara etc. because of the perceived threat.