Is there a reason for the, what seem to be, rather long headtubes?
I think it's mostly down to the relatively high bottom brackets of the frames. UK builders generally copied the long slack Repack bikes first (Saracen/Evans style), but then around 1985-6 we have a brief flowering of UK specific geometry like this, then by 1990 the race scene dominates, going all SoCal on us: lowering BB and bars, shortening chainstays and lengthens toptubes and stems...
BB height looks to be around 12.5". To get the relative positions of saddle, bars and cranks right, the headtube needs that little bit added. Internationally this is more like the early Pacific NW Konas
and Cannondale Beast of the East
See also some of the early Bromwich and Zinn frames and my '86 Overbury's Pioneer
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse