Talkinpeace, it's more complex than that (and not at all about Catholic women having lots of babies).
Poland has a staggeringly low birthrate per woman. See here. Like the other post-Catholic countries (Italy, Spain, to an extent Portugal) the birthrate collapsed as soon as the church's influence ended. There's no particular reason to believe that Polish women in the UK are any more enthusiastic about having children their stay-at-home sisters.
No, the reason why immigrants groups have high rates of children is not just about how many babies each woman has. It's also about the demographics of the immigrant group. In the case of A8 states that joined the EU in 2004, the people that emigrated (particularly to the UK, as it did not impose transitional controls) were almost exclusively of child-bearing age. Those that stayed home were disproportionately older.
So if you take 1000 people born in the UK, they will cover the whole spectrum of ages, with the largest single subgroup being those born in 1964, a slow ramp up to that from 1945 and down to 1995, etc. But if you take 1000 people born in Poland who are now living in the UK, very few will be over forty. The fertile women aren't having any more women than the locals of the same age, and probably fewer; but there are more fertile women in any 1000 people.
I doubt there are enough service personnel to make a significant difference, but the same argument would apply: most servicewomen, and wives of serviceman, are of child-bearing age, and therefore they will have more children than the same number of people drawn from the general population.
There are immigrant groups who have a markedly higher TFR, such as the Pakistani and Bangladeshi community, although their age profile is broader (a lot of older people). But that is not the case with Polish, or (so far as I recall) any of the other A8 countries.