To complement the new features of Simutrans-Experimental (http://forum.simutrans.com/index.php?topic=1302.msg12596#msg12596), I have been working on a modified version of PakBritain ("PakBritain-Experimental") that makes use of some of the new pakset specific features. I have now released the sources, which are available on Github here (http://forum.simutrans.com/index.php?topic=1302.msg12596#msg12596).
At present, there are only a small number of modifications made largely for testing purposes, but, in due course, I plan for this to become a fully-featured branch of PakBritain, and to be released alongside Simutrans-Experimental as its default pakset.