Been giving my MK Indy Fireblade a good going over, replaced the rubber gasket in the water pump that was seeping and decided to buy some decent coolant and bought Silkolene Pro Cool at £7.45 a ltr. Which is cheap to some I have seen online. However, when I was putting it in it look just like water with blue anti-freeze, why is it so expensive as it got other additives and I just a need a cup full to top it up can I use a drop of distilled water.

It is 50% distilled water, so yes you can top up with distilled water, but you will be reducing the concentration of anti-freeze - with a car engine the volume is large enough that there would be little difference, but I don't know how much is in your system. On the other hand, it doesn't really get cold enough in the UK to be a problem, even with a fairly large dilution.