Yes, you're right, Eirwin, apologies, and if Green Frog say it can house that many I'm sure it will be fine. One less problem!
And if you are happy to raise them in the conservatory, and provide heat, and maybe light as well in the shortest days, they will be OK, -but I still think they would do better with the benefit of Springtime sunshine, because of the Vitamin D it provides, as well as the stimulation of sunlight on the pituitary gland. Maybe a little Cod Liver Oil on their growers pellets when they get to that stage? This growing time is the basis of healthy development throughout their whole life, and I feel there's not much point in trying to fight Nature by not waiting until conditions improve for them. Much less hard work for you as well!
The Black Rock name isn't really a big issue, I just wanted to be sure you knew what you were getting.
Chicks hatched in February-March could be expected to feather up enough to go outside for a few hours in a sheltered run on fine warm days at around 5-6 weeks old, but if the weather suddenly changed would be in danger of getting chilled by cold winds or rain. It wouldn't be worth risking this, having put so much effort and care into raising them. Its really good not to hatch until April, to make it more likely that they would get warmer temperatures, especially at night. Putting them out would probably be out of the question in December or January, when yours would have reached this age. Adult chickens are very hardy birds, once fully grown, well- feathered and acclimatised to local conditions, in fact they deal better with cold conditions than with summer heat, but this isn't so for chicks, not yet fully feathered and used to warmer conditions. Adult birds never need heating, so long as they have dry, windproof shelter and liquid water, as you say.
I expect you're the same sort of person as me, when I get an idea I want it all to happen yesterday, but with living creatures it may be a good idea to consider when is the natural time for giving them the best possible start.