I'm in Leicester and had two in pots outside year round for about two years, now they've been in the ground in a very sunny but sheltered spot for three years. Never wrapped in winter but they only flowered last year when we had all that heat.
Mine stays out throughout the year ; sheltered border protected by high hedges from biting 'easterlies' ; flowering prolifically this year following the heatwave .
Live in E . Lincolnshire where winters are normally relatively mild .
It survived admirably last years 'Beast from the East' with no apparent damage .
I had some 50 pale pink 15' oleanders on the Gulf Coast of Texas. They were direct planted in the ground by a previous owner of the home. Covering when we had our few freezes each winter was just not possible. And these tropical trees do not tolerate freeze well, even very established oleanders. Needless to say, mine all died back every freeze we had over the 12 years I lived there. In early spring, I just cut off the dead branches right to the bottom of the central hub (which was most of the branches) and new ones would shoot out very quickly. Aphids are bad on the new young shoots, but I just blasted them daily with the water hose. I eventually set out some ladybirds I purchased. They would regain their entire 15' height in just once season, even the year I cut them down to 12" stumps. Very hardy, these. You just can't prune them too hard as they seem to thrive on pruning. The more aggressively you prune them, the more branches, the more blooms. Mine got prettier and prettier every year.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.