If I brought a dog whose got his pet passport back in the country via Manchester Airport, is it true that they have to do some kind of quarantine?

Why is it, that you can only bring a dog back into the country in the south without having to do this?

There are set entry points into this country. These entry point's have all the facilities to check the animals coming through and impound them if their documentation is incorrect. Check with DEFRA for recognised carriers and entry points.