Hmmm, i just got a huge 50 litre bag of potting soil from lidl for 1.99, and the quality isnt bad, probably not as good as the 40 litre bag i got from wilkinsons that was 2.99, i think basically uou pay for you get, miracle grow and john innes soil will probably be of the highest quality m8, but i've heard that the john innes soil sometimes has too many nutes and can burn the roots of a new plant. I am a noob to be fair so probably not the best person to reply, but i would say if you are goign to b and q then the familiar advertised tv brands are probably best, but saying that if you are goign to be feeding the plants plenty of nutes at a later date then a cheap soil could be just as good, depends on texture really, a nice fluffy soil would do the trick.