I am 182cm with c.90cm legs. My Bob Jackson was supposed to be custom but my Roberts that really was custom has a shorter top tube. I replaced the stem on the BJ, I never had to change a thing on the Roberts.
"you'd have to get very lucky with a used custom frame made for someone equally freaky."
My 1987 British Eagle would never have been custom yet it fits me perfectly, having a 62cm frame with a relatively short top tube. It was made by a small workshop (Eagle Cycle Works, Mochdre, Newtown, Powys SY16 4LD) employing Welsh ex-coal miners and Barry Hoban was their design consultant.
In the mid-80s a quite a number of UK frame makers operated and could offer a variety of sizings. I bought my first good bike(531 frame) in 1985. I went to the shop in The Strand, they measured me up and told me to come back in a couple of hours! When I returned, there was my bike, ready to go and I rode it to Chelmsford station where my car was parked. I guess they had a stack of ready-made frames, all sizes, and just needed to stick the components on. When it got nicked I bought the British Eagle Touristique off ebay for £30 (and spent loads more doing it up).