I know some GWR footbridges could be quite elaborate, but not all of them were! Wantage Road, for instance was a plain plate girder structure, similar to this one at Southam Road http://www.warwickshirerailways.com/gwr/gwrsrh290.htm. If doing it in 4mm/ft, you might even manage to use a Wills varigirder kit! Just make sure you don't have a powerful landowner nearby who has to be placated by an elaborate bit of ironwork.
In addition to the points made above, one other point to remember about whether an overbridge or subway would be used is that the footbridge would not be subject to any great loading, so could be a cheap structure. Any subway would have to be built strong enough to withstand the weight of passing locomotives and would consequently be a much more robust and costly structure.