According to the 2017 Yearbook there was an important reduction of publishers in Poland (over 2,000) and small increases in countries like UK, Ireland, Germany and Norway. I thought that there might be something more than just people leaving JW in eastern Europe and joining in Western Europe, so I checked in jw.org the number of Polish language congregations outside of Poland. As you can see below Germany, the UK, Ireland and Norway have a lot of Polish congregations. Maybe a lot of people is leaving and others are just moving to a different country.
In any case, if western European countries are enjoining small increases I think it is due to JW moving from somewhere and not because of new converts. What do you think?