I need the same thing, and also in the UK, purely as I need to run longer leads to the coilpacks I'm running. 1.2m for the longest few leads...
Stock boots/caps are 150mm tall, and I can't find anything like that, find 120mm max, which would still be far from ideal for pushing them securely on to the plugs, and the stock ones don't seem to come apart to re-use them even if I wanted to.