As has already been pointed out to the LIst, it was simply common practice in the UK in the 50s and 60s to ride fixed gear. As to the reasons these are numerous and varied.
I was not unusual in those days to just have one bike on which you did all your riding ie to and from work or school, for your club riding, for your racing and for your touring holiday. Cycling was the sport, by and large, of the working class...not the "life-style ""statement that it has become recently ie. you've got the BMW, or Ford .or Porsche, or GM, or Honda SUV, so you've got to have the bikes to hang on the back of it..and of course there's that whole healthy living thing. Cycling was your means of essential transport that took you to your place of work, and it was your means of getting away on holiday.
It wasn't surprising, therefore that your one bike took on a lot of different disguises, more often than not as a fixed-wheel bike, but for touring purpose derailleurs were popular particularly with a double chainwheel.. For time-trialling it was often on fixed, but for massed-start it had its derailleurs on again.Mudguards, paniers and saddle bags were simply added as required..and then taken off again.
As soon as autumn started the fixed wheels were ridden all the time. There were several reasons, one of which was that in the wetter weather to be expected at that time of year there was much more grit and dirt on the roads, so fixed gears were simpler and far less expensive to maintain. There was also the feeling that riding a fixed gear gave the rider better flexibility and bike-handling skills while at the same time enabling a superior style of pedalling -"ankling" - to be developed. Even touring in winter would be done on fixed wheel.
As for the question of just how fast you allowed yourself to pedal downhill, it was all a matter of common sense and as soon as you felt yourself being tossed up and down in the saddle, you applied the one and only brake - the front one- to regain full control of the bike.
Another factor that determined the speed of pedalling on a descent was the amount of faith that you had in your transmission. In those days most of the chainsets were made of steel, and if you did have a TA , Stronglight or Gnutti or something equally exotic you didn't use it in winter. Many chainsets and rings were pressed rather than turned resulting in the rings often being both slightly out of true and out of round.Bearing in mind the requirement to have about 1/2" of up-and-down slack in your chain, it was often very difficult to get anywhere near the correct tension when using a slightly oval ring. Trying to get that same chain to run straight from front to back on a slightly bent chainring was often asking the impossible. So we adopted the use of the small fork - Oscar Egg Super Champion style device - fixed on the seat-tube through which the chain had to pass. Failing one of these it was not unusual to see a fixed-wheel bike with a Simplex or Benelux " double clanger in place, withits cage screwed tight in order to try to direct a rapidly moving chain onto an often elusive chainring.
If you have ever had the bad luck while descending a hill rapidly on fixed gear, to have your chain jump off the chainring, leaving you entirely dependant on that one front brake..and remember these were not your dual-pivot short-reach jobs..they were long and thin and flexible GBs or Weinmann 730s and the like..then you rapidly learn an important lesson about how you SHOULD descend on fixed gear. We didn't have helmets and track-mitts either in those days, just cloth caps and woollen gloves.
One of the finest sights I have seen in years was in the last Paris-Brest in which half-a-dozen or so British club riders simply took their everyday fixed wheel commuter bikes, saddle-bags and all to the event and simply pedalled on those single gears all those 1200+ kilometres. There is simply no finer sight in cycling than riding behind a cyclist ankling his 67" fixed gear with consumate ease. But there again there must be an awful lot of folks who just think what a sad sod I must be....
Norris Lockley...ankling his way to bed..Settle Uk