The UK has very strict requirements around certain foods that their farmers have to meet. Allowing US farmers to bypass those requirements will undercut those domestic farmers and while individual consumers may still opt for the more expensive domestic option, restaurants and takeout businesses will usually opt for the cheapest option which introduces it to the country even if the consumers don't want it. It's not like McDonalds has a sign that says "Made with US Chicken!" on your nuggets. Secondly, allowing cheap US chicken just penalizes the poorest in your country that will go for the cheapest option to survive. Some people don't have choice and food standards help bring the bottom up.
>restaurants and takeout businesses will usually opt for the cheapest option which introduces it to the country even if the consumers don't want it
Restaurants are free to advertise that they use domestic chicken. You can even legislate mandatory labeling if you're so inclined. The fact that you think consumers need to be actively prevented from getting US chicken, because they don't have the capacity to decide for themselves contradicts your claim that "Brits are NOT having it".
> It's not like McDonalds has a sign that says "Made with US Chicken!" on your nuggets.
I'm not sure whether this is still the case, but the cardboard boxes that McDonald's packages its burgers in in Australia used to have '100% Aussie beef' printed on them, but the beef was actually imported from South America by a wholly-McDonald's-owned subsidiary called '100% Aussie'.