I have just noticed that 10 kg bags of Big K lumpwood charcoal are only £5 from Countrywide Farmers.
If there isn't one near you delivery is only £3.95 for up to 3 bags. So that works out at £18.95 for 30kgs. including delivery
I have used this charcoal and it seems to me to be of at least equal quality as the Makro/Booker Blue bag restaurant grade stuff.
Here's the link if you are interested