"Many Poles live in Great Britain."
Translation:Dużo Polaków mieszka w Wielkiej Brytanii.
Those are polish numerals and quantifiers rules.
most quantifiers (and many numerals) need this kind of construction:
- wielu + noun in genitive+ verb in singular.
dwie dziewczynki idą (numbers that end with dwa, trzy, cztery) two girls walk
pięć dziewczynek idzie (other numbers) 5 girls walk
No. Although a person from "Wielka Brytania" is "Brytyjczyk" (a man) or "Brytyjka" (a woman), although the adjective is "brytyjski", the country is always called "Wielka Brytania".
"Brytania" is used when you refer to Britain as it was during the times of Ancient Rome and its conquests.