It sounds like the o.p. is in construction so I will mention only three coop-specific things in that regard:
1) chickens require a lot of ventilation for good health, figure out the best design so you can build it into the coop in the most functional way from the beginning rather than having to retrofit (see my ventilation page, link in .sig below, for suggestions); also
2) the popdoor, the little door the chickens walk thru to go outside?, cut it at least 6-10" above the actual coop floor, so there's that depth of sill. This will keep shavings or whatever else you're using as floor litter from getting continually kicked out the pophole. Some do this for the people door as well but in that case it should be (instead) a removable sill so that you can still get a wheelbarrow in there when you want to. And
3) [most] chickenwire is not predatorproof. Predators are WAY stronger and more determined than people tend to expect. Even "just" a raccoon is extremely strong, and a spiderman-like climber to boot. And dogs and other things will dig under the foot of a run fence. I won't go into it all here, but look into Yer Better Type Methods Of Predatorproofing and invest in them. It's worthwhile.
4) The biggest strictly chicken-related mistake I see is people getting caught up with religious fervor in this "2-4 sq ft per chicken indoors, 8-10 sq ft per chicken in the run" thing. That is, at best, a
minimum, designed to avoid high likelihood of cannibalism. But it ignores climate -- which matters a LOT -- and it ignores that a person might be trying to have happy low-stress natural-behaving chickens. Really IMO it is waaaay best to build the biggest coop you can manage, and then stock it with the FEWEST chickens you can content yourself with.
Breed is not a big deal, except that you should pick one suited to your climate and most people should pick one with a calm temperament. Being as you are in the UK this probably doesn't rule out very much at all
, so pick whatever you can easily obtain from a good source (healthy stock) and happens to "float your boat". If you are serious about economically-efficient egg production, sexlinks and production white leghorns are the only real options; but if this is just recreational, all breeds lay "somewhat" and most commonly-available ones lay pretty decently for family kitchen purposes.
Good luck, have fun,
Pat