Congratulations on your new chickens! I can guarantee you'll be as obsessed with them as the rest of us are, in just a few days!
...They are coming with bedding and feed, the area I want to put the coop and run is very overgrown with grass/weeds, will they like that or not?
They will love it. Let them have free run of all the weeds and they will eat them up, then turn over the soil searching for weed seeds! Chickens are the best little garden assistants ever!
Are there any plants that are poisonous?
Yes there are. Local people will be able to advise you better, but for me I had a lot of nightshades in the run before I got my chickens. I started pulling them out but didn't get them all. I was surprised (and pleased) to find that my new little pullets who were only about 12 weeks old at the time, ate all the weeds that were good for them and left the poisonous ones alone!
Of course if you leave them confined and the only greens left in their run are poisonous, they may over-indulge in them. But in my experience, if they have plenty of choice, they will avoid the poisonous plants.
My allotment is 50ft x 30ft I know my crops won't be entirely safe and things like the strawberry patch which is netted over anyway will that be enough to stop them?
If your chickens can jump on top of the netting and press it down, they will eat everything they can get their beaks on! I think you will need something much sturdier to protect your strawberries.
I read a thread on here about poo picking - I thought chicken poo was good for the garden, if they poo on the allotment I wasn't planning on picking it up - or should I?
Personally I never poo pick. I just let the chickens dig it all into the garden beds. But I am also surprised how much poo is produced by such small animals!
I would suggest you work it out as you go. If you're happy with them digging it in, then let them do it. If you're finding the poo is a problem, pick it up and throw it on your compost.