If the Cross River tram in London not been cancelled, it would have followed part of the route of the Kingsway Subway (although think the plan was for it to run at street level, and might have led to the removal of the remaining visible part of the Subway) and some street running along former tram routes.
Well it would be fun at least to see a modern tram try and turn on an old tram terminus alignment (many of which survived into the trolleybus and motorbus era as useful turning circles, albeit alas many no longer even served).
Don't think there were many tram turning circles in the UK on first generation tramways - Blackpool was an outlier in this. Generally speaking, trams got to a dead end or crossover and then changed ends. Most of the turning circles I'm aware of were built for trolleybuses not trams, and in some cases a route had to be extended or curtailed to fit where a turning circle could be arranged.
London had a few loop (rather than turning circle) terminus arrangements, many south London routes ran along Victoria Embankment going via Westminster then Blackfriars Bridge (or vice versa), some routes did a loop at the suburban end via Brixton, Streatham, Tooting and Clapham (or vice versa) - the night service at least ran as a figure of eight from the Embankment.
There was also a one way loop to and from Beresford Square in Woolwich for trams from the Eltham direction (some of the tracks stayed visible in to the 80s at least.) I think the only turning circle (as such) in London was at Tooting, and came out of regular use when trailer car operation ended in the 1920s (and think it was later lifted.)
There were a few single ended trams that ran between Sheffield and Rotherham, I think this route had turning circles at each end, but again an outlier, and (I think) the only single ended trams in the UK.