The question of whether British Standards should be free, provided 'at cost' or sold at a profit is a separate issue.
The actual cost of printing a paper copy of a document of a few hundred pages is, today, fairly trivial - so, even if the cost of providing access to an electronic copy is 'orders of magnitude less', the difference between the actual cost of making paper or electronic copies available would also be fairly trivial. So long as the selling price of the paper copy is (for whatever reason) the printing cost +£X (which is roughly £X in total, since that represents such a high proportion of the selling price), I see no logical reason why the price of an electronic copy should not also be roughly £X.
As above, the argument that £X should be very much lower, maybe even zero, is a separate one, but not one which I think should differ between paper and electronic copies - the cost of which therefore ought to be fairly similar (however high or low).