Okay...put away the idea that chicks need lots of heat, especially once they have feathered in well.
My first batches, I carefully, carefully, brooded with heat lamps and slowly, slowly integrated them outside fearful they would get too cold in our constant damp. (You've got the UK....I've got the great Northwet).
Since you've been holding them in the house, they will need to acclimate to the outdoor temps just like a hot house plant needs to acclimate. They should be off all heat in the house now, and you can let them outside during the day....they probably would be fine over night too...but I would work up to that over a week.
As to why I realize chicks don't need heat...I ended up burning my first coop down with my fear that birds can't take cold (put up a flood lamp during a really cold snap). So, not wanting to repeat that, I went to natural brooding with hens.
My first brooding hen was an experienced Silkie. She taught ME what a good broody hen can do. I ingnored her first brood in NOVEMBER of all times...but she came back with renewed diligence in FEBRUARY. I caved and got some friends eggs (I didn't have a rooster at the time). It was so cold that day I feared the eggs might be frozen as they were left on the porch.
I put them under her, she sat. 21 days later 3 beautiful chicks hatched. To my amazement, they ran around in cold winter air from about day 2 (we were averaging about 32 at night, about 40 in the day time, with rain). All I did was make sure they were out of direct weather and had a dry run.
Those little dears stayed cozy in their warm little down jackets using momma from time to time as a warming hutch. It broke me forever from thinking that little baby chicks had to be carefully heat brooded for weeks. I've learned that actually messes with their systems. (I've since had hens hatch chicks in horrid snow storms with temps dropping into the sub zero areas and frightful wind chill...yup...no direct weather...chicks are out in the blown snow with momma scratching....it will blow your mind).
Nothing wrong with artificial brooding as we all can't wait for the timing of a hen, but you absolutely don't need weeks of hot lamps. (Actually I think
@Blooie has a great heat pad method that mimics a brooding hen really well and avoids the pit falls of heat lamps).
So my birds hang out in the Oregon rain...and I mean all of it...the drizzle, sprinkles, steady, blustering...you know....constant type...they just have a nice dry coop at night where they can re-preen and oil their feathers (which act like a really nice oiled rain slicker).
So do harden them up with your day time temps, then get them into an unheated but weather protected coop and leave them there. They will do fine if they have enough ventilation and no direct rain or drafts.
Congratulations on your new flock
LofMc
