I've looked further at the plans and can see how a Burlington chassis could be classed as "modified" - rear boot outriggers cut off, front legs shortened, sides tapered so accept there is no argument (with DVLA) that per the rules IVA due.
I THINK, however, it'd be possible to build a car in the style of Burlington / Midge on an unaltered chassis. Running boards would hide the original full width side rails. Front and rear would just have to have ugly trim / bumpers added - until inspected by DVLA and then who knows what might happen to them
Paul H