Your climate is similar to mine in Western Oregon, meaning it's not frightfully cold during the winter. But if you have truly young chicks without full feathering you're going to have to keep them warm. A brooding box with a infrared light bulb is a very common setup. Or you can keep them indoors in large box or tub or whatever. It's not convenient for you but your chicks will be comfortable. At about 6 weeks of age you could move them into a colder area but not exposed to freezing temperatures or cold rain. At about 3 months (I'm estimating) they should be able to tolerate a London winter as long as they have a coop of some sort to sleep in, and a place to retreat from the cold rain.
As far as which breeds, there arn't any whose baby chicks can be released outdoors without serious risk of disease or freezing. But once fully feathered and acclimated to the outdoors, any breed should be suitable for your climate.