I'm not an immigrant but have lived abroad for years in the past and am married to an immigrant.
Bedtimes - yes British bedtimes do seem early compared to other countries but I do wonder how kids cope with less sleep. Currently staying with in laws in southern Europe and my kids are getting to bed around 11pm every night. I try and be relaxed about this but my 6 year old just can't cope. He's having frequent meltdowns and behaving badly. I was an au pair in Spain and the kids always went to bed well after 10pm. We had to literally drag them out of bed in the morning for school and they'd be groggy and miserable . They had a 2 hour lunch break at school and I'd bring them home for lunch. They would often fall asleep after lunch and then I'd have to wake them up and drag them off to school again. I am not convinced late bedtimes are OK for young kids!
Only a couple of people have mentioned the binge drinking culture in the UK. I find it quite appalling to see people drinking and vomiting in the streets. I've never seen this behaviour in other countries (well,only seen Brits do it). I used to drink like this myself and really find it repugnant now I've stopped.
The other thing I dislike is the class system. Yes you see rich and poor people in other countries but here the class system is so much more than money. It relates to your entire lifestyle - how you speak, the food you eat, the clothes you wear, the holidays you have, the names you give your children. There's such a huge divide in the UK. It's like people from different classes can't understand or relate to each other at all.
Apart from that there's so much I love about my country!
My immigrant husband struggles with the lack of bidets. He finds it disgusting that we don't wash our bums with soap and water after going to the toilet! He hates the fact there are still sometimes separate cold and hot taps (I agree with this. Not something I'd ever considered before he pointed it out). He thinks lunch should be the same as dinner - a proper sit down hot meal. Never a sandwich!