Type or colour has nothing to do with it's composition, as there are no standards for it.
Unless you know which brand/bottle is already in it there is no telling if you're getting something of the same composition. Every brand has it's own recipe. The colour has nothing to do with what's in there. It's usually just a dye.
If you just need a litre or so, you can always use demineralized water.
Coolant is basically water (maybe even up to 80-90%), glycol (a type of alcohol) and corrosion inhibitors and ph balance salts.
Mixing different corrosion inhibitors may interfere with their workings.
The amount of water in the mixture usually is a measure on how low the freezing temperature will be. The uk usually isnt that cold, so adding a little bit of water isnt a real issue imho.
But changing the coolant once in a while (every 3-5 years) with properly draining the system is always a good idea.
Try to get as much of the old coolant out of your system, for example by unhooking the two radiator hoses and blowing compressed air in one side to get the residue out (both out of the radiator but also out of the block).
There is also an extra drain plug on the side of the block (best accessible on a car lift I think).
But getting everything out is a lot of work.
Coolant usually gets corrosive on the spots where there isn't much flow. That happens because once corrosion starts, the corrosion process itself leaves substances in the fluid that can make the fluid more corrosive. When there's a lot of flow those substances are diluted in the larger amount of coolant but when there isnt much flow the concentrations keep rising.
