This high immigration is reflected in the increasing numbers of
foreign language groups and congregations in Britain. For example...
Polish - 32
That seems a significant drop post-brexit?
EDIT: I think you've mis-read the jw.org search facility?
This is my list of Polish congregations (excluding groups attached to other congregations)
1: Aberdeen, Polish
2: Bedford, Polish
3: Bicester, Polish
4: Birmingham, Polish
5: Bournemouth, Polish
6: Bracknell, Polish
7: Brighton, Polish
8: Chester, South
9: Corby, Polish Central
10: Coventry, Polish
11: Coventry, Polish
12: Crawley, Polish
13: Dartford, Polish
14: Derby, Polish
15: Edinburgh, Polish
16: Glasgow, Polish North
17: Glasgow, Polish South
18: Hull, Polish
19: Inverness, Polish
20: King's Lynn, Polish
21: Leeds, Polish
22: Leicester, Polish
23: Liverpool, Polish
24: London, Polish Central
25: London, Polish Debden
26: London, Polish East
27: London, Polish Hillingdon
28: London, Polish North
29: London, Polish Southbury
30: London, Polish Streatham
31: London, Polish West
32: London, Polish Wimbledon
33: Luton, Polish
34: Maltby, Polish
35: Manchester, Polish North
36: Manchester, Polish South
37: Milton Keynes, Polish
38: Morley, Polish
39: Newcastle Upon Tyne, Polish
40: Northampton, Polish
41: Nottingham, Polish
42: Peterborough, Polish
43: Rugby, Polish
44: Rugby, Polish
45: Slough, Polish
46: Southampton, Polish
47: West Bromwich, Polish