what length of cables do I want to use for the throttle, clutch and front brake?
While Triumph listed shorter cables for "UK Spec Bars", certainly to start with, I advise using the 'US Spec' cables that're on the bike already.
Reason I advise this:-
. You'll note that the existing cables curve around to the lever or twistgrip from the beside the frame steering head on the same side as the lever or twistgrip? At the risk of telling you something you know already, smooth, large-radius bends minimise any friction that manifests itself as heavy lever/twistgrip operation.
. With "UK Spec Bars", the levers and twistgrip will be closer to the frame steering head. If you use shorter cables and curve them from the same side of the frame steering head as the lever or twistrip, you'll have much smaller-radius bends, especially when the steering is turned to full lock on one side or the other.
. However, if you continue to use the existing 'US Spec' cables but route them around to the opposite side of the frame steering head from the lever or twistgrip
, the existing smooth, large-radius bends will be maintained.
Fwiw, I've had two T160's for many years, one fitted with US-spec. 'bars and one with much narrower 'bars without any 'rise' (Norman Hyde 'M'-bars). Each 'style' suits me for different purposes so, if I find that one bike is doing significantly more/less mileage than the other, I swap the 'bars (and footrests) between the bikes to change the 'style'. I've always used 'US-length' cables on both bikes, simply routeing them on different sides of the frame steering head as part of the swap.
Also, if when you try the "UK Spec Bars", you don't like the riding position (I didn't and I use the Hyde 'bars with reaset footrests), and you decide either to refit the 'US Spec Bars' or try some other ones, you won't have wasted any money on cables you'll never use.