You really are into money-saving, aren't you, Ed?
Washing-up liquids, shower gels, shampoos and many household cleaning products are basically the same. i.e. they start with a 'bog-standard' detergent, add a thickening agent (such as salt) and chuck in a few things to make it look and smell nice. In many cases, it's only the ratio of such 'extras' which distinguishes between the products.
A recent television programme on allergies suggested that one part of the cause of the sudden rise in allergies is that we're now removing some of skin's natural protection by too much washing and the use of lots of extra chemicals in the products we put onto our skin. It would therefore seem to make sense to go back to using unadulterated soaps or detergents which have got the fewest 'extra' in them. The best (and cheapest) way to buy such detergents is to buy the cheapest washing-up liquids. (My local Tesco has recently withdrawn their 'Value' brand but buying the cheapest available will be perfectly good enough for both washing-up and showering).
So (because there are fewer 'extras') it might actually be better to shower with washing-up liquid, rather than using shower gel. However, Tesco 'Value' bath soap is really cheap, and will do the job just as well.
Chris