for me it depends on what I am doing! If I am just going about my every day business then im ok up to about 23c I would say, can manage ok in mid 20's but 26c above would generally be too hot. If I am doing something more active like mowing the lawn the the mid teens would be nice. Also depends on the type of weather, if there is a nice breeze then I can tolerate warmer temps. I live smack bang in the middle of central England which is pretty nasty on hot summers days, humid and sticky, no wind and warm overnight, its actually the overnight temperatures that I dislike more than the daytime. As others have said, the cold temperatures are not so much of a problem, decent clothing should always be good enough to keep you warm in this country.