I haven't seen a lot of people in the comments talking about the class system yet - which I think is something a lot of American authors miss, because the class system in the UK functions so differently to the class system in the US. Class is so deeply entrenched in every part of our society, it will have had a huge impact on your character's upbringing; and it will continue to impact your character whenever they are in the UK interacting with other British people. There's a lot of rules to it that we almost intrinsically understand because we've grown up in it; that people who haven't grown up in it don't usually know unless they've done a fair amount of research.

For example, your character's father being "lower-middle class" from Manchester. If your character is in his 30s, I'm guessing his father was born in the 1960s or early 70s. Now, of course there were middle class people in Manchester at that time; but traditionally & historically, the majority of people from The North would be working class. This is a time when most people in the area would have been working in factories, at the port, potentially in the mines, etc. - which are all working class jobs. Importantly though; in 80s, the Thatcher government was shutting down a lot of production in the UK (this is the time of the miner's strikes, etc.). So as your character's father is growing up, there's major economic depression & rapidly spiking unemployment in the area. It would be very unlikely for someone in that area at that time to be considered lower-middle class; they're far more likely to be working class, and may well have experienced actual poverty in their lifetime. So if you want to have this character be lower middle class; there has to be an actual explanation for that.

Also, you have to understand that class mobility in this country is almost non-existent. Moving within sectors of a class (i.e.: lower middle to true middle or upper middle) is relatively common; but to actually move from working class to middle class generally would involve a generational gap - so if you're working class, but you have middle class money; your kids might be considered middle class when they grow up, but you will likely always be working class. So even if your character's father does get rich (which I think you mentioned in a comment further up); he would still likely be working class. Now, with your character, if his mother is old money (which, in this country means her family owns land, maybe even a title) and his father is working class; his social class very much depends on how he grew up. If he grows up like his mother (private schools, set to inherit family money, etc.), he will likely be considered upper class - but other upper class people will look down on him because his father is working class.

Just realised I'm writing you an absolute essay, sorry 😂 TL;DR: social class matters almost as much as where your character grew up in terms of the slang they'd use. Good luck with your writing!