Nightly development builds of Pak128.Britain-Ex are now available. They should be extracted into the ~/simutrans/pak128.Britain-Ex directory (create one if it does not exist). They can be found here:
Nightly pakset builds (http://bridgewater-brunel.me.uk/downloads/nightly/pakset)
The nightly builds are currently using the half-heights (https://github.com/jamespetts/simutrans-pak128.britain/tree/half-heights) branch on Github.
See here (http://forum.simutrans.com/index.php?topic=15886) for more information about nightly builds of the game executable.
Doesn't work.
There seem to be some problems with the DNS for the Bridgewater-Brunel server at present. Until these are resolved, I suggest using http://46.32.231.222/downloads/nightly/ (http://46.32.231.222/downloads/nightly/).