I am a relatively new chicken keeper as I have only been keeping them for about 3 years. I am sure you will get many replies from very much more experienced chicken keepers on this forum. I can assure you of one thing, however. you will not stop at two -or possibly even four. Chicken keeping is addictive! I now have five and the only constraint on me is the size of my coop and run. I would strongly recommend that you think ahead in this regard and have accommodation for more than two, even at this stage.
Secondly, just to reassure you about dogs, we have two chocolate Labs who are completely accepting of the chickens and never bother or chase them when they are all out in the garden together. We put them on leads and held them when we let the chickens out for the first time. Every time they tried to move away towards the chickens, the leads were given a gentle tug and they got a firm "no." That was it. Our neighbour who also keeps chickens, did this with their beagle and she too has never tried to harm either their birds or ours. However as you rightly say, some breeds of chickens are less placid and are best avoided.
I'm not quite sure what you mean by British breeds so perhaps my recommendation is not really what you are looking for. My five are a Blackrock, a Welsummer and three pure breed Rhode Island Reds (two large fowl and one bantam), my all-time favourite breed. RIRs are friendly, calm, good-natured and beautiful. They are also good layers. What's not to love about them? (I would not recommend Welsummers as my one has never really become tame. She's always been a slightly nervous, skittish bird. That has been my experience anyway.) As far as adding new birds is concerned, I would never add a single bird. I would always add two at a time.
Finally, do look up what Hillfooter has to say about starting chicken keeping. You will find plenty of thoughtful, practical advice.