Buttsy
Established Member
Pontypool & New Inn - the island platform is very wide there, or have I got the wrong end of the stick??? 
Yes, so really the answer to the question is to find the station with the narrowest single island platform.
I'd nominate one of the Glasgow Underground stations, although as they are below ground level, the circumference of the earth will be slightly less...

As far as the LSWR mainline goes, Farnborough Main is slightly wider than most of the original island platform stations. Hook and Winchfield have had their islands removed and the remaining gap is about 21.5m, likewise Walton on Thames and Esher have islands remaining but out of use, the overall gap between the outer platform edges is again about 21.5 m. (Google Earth accuracy!)
Is it possible Farnborough is slightly wider than the norm for the line because it has a more significant curve?
Out of interest how wide is the gap at Eastleigh??
My local, Hook should be a contender. Width of Farnborough, but I think the gap left by the former island is bigger. I'll go down the road with a tape measure, shall I?
Or why not read the previous few posts? I've highlighted the relevant info...