Unfortunately it has been almost a boat-free summer, and my lab is being rebuilt
The sat data for UK is on a ~90mx90m grid, so a a 30kmx30km area would be ~300x300 real data points. for a 600x600mm cut, that is a point every 2mm. BigTopo can try to interpolate a finer grid if you want, and the stl file can be smoothed if faceting is visible.
As a free program, BigTopo is written to use freely available data files. It cannot read other formats. More file flexibility would be nice, but that might be moving towards a specialized product, perhaps a commercial one. Hmmm, retirement looms ...
If anyone knows of other, free data sources that cover the globe, I would see about adapting BigTopo.
Dave, can you tell me the area that you are interested in ? I will see what I can do ...