Best coolant under £8/L
grocery/pharmaceutical distilled water and kill coil. Best thermal performance as well.
1 Ltr = 1 cc = 1KG, though thats what has been assumed in Physics all along. The additional weight maybe from the packaging.
Id take his advice and run... distilled water will be the cheapest performance fluid you will get.
The reason some coolant may be more efficient than distilled water is the additives.
Your paying extra for these additives...
Your paying £3.50/£5.00 extra for 0.5-1.5*C worth of cooling difference
rest is up to you