Shorts & T-Shirts here all the way through school... jumpers allowed if cold(ish) outside... too cold and we don't do anything.
As for the PE in pants/vest or bare chested, yes, there are several schools in this area that also, with the 'young uns' do pe in underwear and indoor games in shorts/plymsols only for older (yr 3+) (<--boys, girls required to wear a vest).
I seem to recall a similar thing popping up on 'the other' forum, centred on a Welsh school, with a variety of explanations given, including, time lost from lesson in changing, cost of separate pe kit, health & safety issues around 'loose' clothing.