The seat tube lengths given on the Cannondale website appear to be "virtual" lengths therefore, the offering is as follows:

63.5cm
58.4cm
53.3cm
48.3cm
43.2cm

You would likely pick the size closest to the size that you currently ride. I am not that familiar with the Cannondale sizing methodolgy so, if someone provides different information on sizing or fitting these bikes then I respectfully defer to the higher authority.

Another bike you should consider being that you reside in England is the "Mercian". These bikes look excellent and are made for the long road.

http://www.merciancycles.com/frame_king_mercia.asp