Chemistry's good but I wouldn't say the A-level I did really prepared me for much of my engineering course! Some of the stuff on bonding and structure is relevant to materials but that's about it - the thermodynamics we learnt in Chemistry was very different to that in our course. I'd say Computing would be a good choice as long as it contains some decent content, ie. proper stuff on programming, digital electronics or communications. (And by communications I mean in an engineering sense, not as in "what are the advantages and disadvantages of using a fax machine")