Late 60's/ early 70's, say 1968 to 1972. If the original colour was gold it'll be an early one, if it was metallic blue or dark silver grey, a later model. Can't recall the model name now, I keep thinking the ten speed was labelled as a "Tour of Britain", but I don't really know why that name persistently keeps coming to mind. Bike shops would keep stock until sold back then, so you could buy a "new" bike in 1971 that had left the factory in 1969. Serial numbers don't help a lot either.

If you build it as a single speed or fixie, it'll be a lot lighter as you'll have no gears, multiple freewheel cluster, shifters or excess chain. Fit as many alloy parts as possible and it should turn into a really good ride. I always put two brakes on my fixies nowadays as I'm not as brave (ot as reckless) as I was back in the day. I also like having two brake hoods to give an additional hand position, aero levers with decent rubber hoods can be found NOS quite cheaply from time to time and I actually prefer not to have loops of cable in front of my face when down on the drops. YMMV.