Having actually done the coast to coast and appreciating its capacity for humour I thought this might be an amusing film. Alas! I have never written a review previously but feel compelled to now.
It was impossible to feel any empathy with the characters involved. Three of them seemed reluctant recruits and showed no sign of the fitness or enthusiasm required. Sole interest was in drinking and over-eating. We were required to believe they had covered the significant distances involved after lazing around until lunchtime, which stretched my credibility too far.
All of this might have been incidental if the film had actually been remotely funny. The "humour" was of the lowest common denominator, gross kind, offensive rather than amusing. We switched off after enduring for an hour when it was apparent no improvement could be expected.
Unless you enjoy the American Pie sort of rubbish avoid like the plague. Spend your money on the Billy Connelly film "What we did on our holidays" instead.