You will probably be OK with 6mm^2, but really you need to check what voltage is at the start of the run preferably with a significant load on the house (such as the immersion heater and electric oven on if you have them) to know what voltage drop is acceptable. Ask your Sparky what they recommend and what discount they will give you for running the cable.
Don't buy cable from B&Q - it's ridiculously expensive. Your Sparky may provide it for you at wholesale prices.
Don't buy cable from B&Q - it's ridiculously expensive. Your Sparky may provide it for you at wholesale prices.