Will they fail to adopt? Almost certainly the answer is yes. Once they flood the UK, they are very likely to find a patch of land, build their small homes and be happy. They will have no interest in learning English language, pay UK tax and follow many British regulations. British regulations on house building, roads, fences, disposal of food, plastic and so on.
Just a small example of problems to come. UK law states that all children must go to school, otherwise parents may be fined. Government bodies responsible for this law will get a nasty taste when they find out that in Romanian culture school is not in the top 10 list of what the child must do.