I can't agree with that. First of all the best educated people can make really good money in Poland, so usually they don't leave and no matter what they say I think that most of those, who left would come back immidietly If they could make here similar money.
Well, if they are lawyers, who passed their bar exam, they can usually make good money and wouldn't be able to practice law in the UK, so yes, they stay in Pl. But doctors, engineers - their skills are more easily transferable. There are graduates in biotechnology or robotics that I know, for whom there are simply no jobs in Poland - we are not exactly a scientific powerhouse, and are still unable to fully take advantage of home-grown scientists and innovators. Poland does not create enough positions in the commercial sector for highly specialized people.
Have a look at Polish -language message boards (Gazeta.pl) and see how many people declare that they do not intend to return to Poland. Once you have children in the UK, it becomes very difficult to transfer them into the Polish educational system, which is totally unprepared for such eventualities.
As I wrote, it is true that the majority of recent immigrants are not those that are the best educated. But quite a few are, and the ones still in Pl continue to be lured by better prospects in the UK - my friend, who is a scientist, has recently been headhunted and is moving here, my cousin, who is an engineer, etc, etc.