immigration checkpoints during a layover?

I was wondering if anyone here might know, if you flew from the USA to Manchester UK, but had a stopover in London before the connecting flight to Manc, would you have to go through the immigration checkpoint in London before boarding the connecting flight, or would that not be until they got to their final destination in Manc? I've never landed in another UK city myself when I used to fly from the US to Manc, so I wasn't sure what they do for passengers that have a connecting flight to another UK city. I ask because my family is coming to visit me for my wedding next month, and they're going to be taking a British Airways flight that stops in London and then getting a connecting flight to Manc, and I was just thinking that it might be likely that they would have to go through immigration in London instead, as the flight from London to Manc would be a domestic one and would probably not be going through the immigration checkpoint once they landed in Manc since it's not an international flight? I was just concerned because I think they will only have a little over an hour in London before their connecting flight is due for takeoff (and that's if the first plane gets in on time... ) and I know from experience that you can spend nearly that long in the Non-EU passport line sometimes. I would hate to think they might miss their connecting flight and get stuck in London while they have to wait to make other arrangements to get to Manc. Is it possible they do things differently for those passengers that have to make a connecting flight, maybe fast-track them through the line somehow or else let them wait till they get to their final destination to go through immigration? Sorry if this is a bit long-winded, I'm just excited for my family's visit but also want to make sure that they arrive here ok thanks in advance to anyone who might know more about this than I do and can tell me something to help me stop worrying so much hopefully

I'm pretty sure they will go through some sort of immigration checkpoint in London. It happened to me, though it wasn't a main checkpoint, just one that processes people getting on connecting flights ( I assume, there was only one line, and they they were processing both EU and Non EU) Once I landed in Manchester, I didn't go through any immigration checkpoints there.  It was really fast- I was only in line 2 minutes and the IO was very quick... Hope that helps?

My experience was the same as Smashley's.  They have a "special" immigration checkpoint thingy for connecting flights.  There was no line when I went through and it was super quick and easy.  There's signs for connecting flights and it directs you to where you need to go...so nothing special to look out for.

Oh that's really good to know! Thank you Smashley and Eastside, I was hoping they would have some system like that for the people that need to hurry on to a connecting flight. I'll let my family know to look out for the line for people with connecting flights, and I guess otherwise if they're not sure where they're going they can find someone to point them in the right direction. I feel better knowing things should be easier for them in London!