Not tried, but I've had a look before. Both were recommended when trying to source vegie friendly whey.

Both stock unflavoured whey 80% concentrate.

Myprotein - 4.5kg = £31.95

Bulkpowders - 5kg = £24.99

Not sure how it works out after p+p from myprotein, but bulkpowders offer 24 and 48 hr shipping which are £4.99 and £6.49 depending on which you choose.

Mixes easy and tastes good.