Albizia julibrissin 'rosea' is the hardiest in this country, but even that needs a very sheltered, sunny position, and may be killed back to the ground in a harsh winter.

Vulnerable to freezing in the pot if we have a winter like last year's, and they do get rather large for a pot eventually, but up to you. If you can move it somewhere frost free for the winter, might be worth a try, but it will get too large for a pot.

you could wrap the pot in bubble wrap and give a straw protection on the top in really cold weather. This is what I have done in the past with my Japanese Acer.

We are in Hampshire and growing the Albizia j Rosea in the ground, it has come through two very hard winters(unheard of in Hampshire) it is very slow growing, and has the minimum amount of protection (a slightly sheltered position) was tempted by the colour of the Chocolate, but had already bought the Rosea, not sure this is of much help?