B&Q seem to be cheaper than Wickes, but the 'shed' prices can be very high. I bought some black angle in B&Q recently, wasn't too expensive and welded very well.
One thing they do is welded square steel tube in loosley 'telescoping' sizes, A bit hard to arc weld without making holes, but handy for space frames. Watch what you get as there are two wall thicknesses.
Neil