B&Q dont do stanless pipe, I would buy flexable plastic stuff, the type you use in domestic plumbing 3/4 should do the job, and get the type that uses plastic fittings too. Over here its gray in colour. add a couple of pipe clips and SS screws for fitting, and you should be away......
PS Irish and English Plumbing sizes and fittings are not the same!
PPS I have never seen a 'real shower unit' but I have seen homemade ones, and pics on the web.
NOT THE SHARPEST KNIFE IN THE DRAWER