Fly SFO to Heathrow and then take the train to Manchester or fly to MAN. What to do?

It seems very risky to have a connecting flight on a separate ticket, no matter which airport you're transiting through. This seems even more risky at LHR. I would not do the flight combinations you've described. I sense that 2 hours in LHR will not be enough time on a separate ticket.

I've avoided LHR, in part because of previous experiences like yours, so I can't speak from experience.

Having already visited London many times, it seems that there's no interest in spending time there for this trip; but if you're wiling to spend a few days there, it would make all of this more feasible. Then you can adjust to UK time and train to Manchester another day. That's what I would do, especially given your previously abysmal experience at LHR. If no interest in a repeat London visit, I would consider the train connection to be the safer way to go.

LHR is a nightmare for connections. I’m not comfortable with a 3 hr connection on the same ticket. There’s no way I would book a flight on a separate ticket with checked bags with less than 3-4 hours connecting time. You have to do everything you said: collect your bags, go through immigration, check your bags, go through security. Most likely change terminals. Possibly go through security again. There is no airside way to do this. It will be as if you arrived to the airport from the outside.

The only carrier from LHR to MAN is BA with typically 9 flights a day.
The train from Euston is every 20 minutes.
The highest fare you can pay on the train from Euston is £79.80 off peak. A walk up fully flexible fare.
The train is the huge!y obvious choice If not on a through air ticket.
If the SFO flight was late, retimed or cancelled and you had to buy a new air ticket for the last leg it may well be north of £200.
From SFO to LHR direct you would fly BA, UA or VS.

But you can also book with BA, so therefore presumably with AA, for an AA flight to JFK, then BA to LHR, then BA to MAN, all on one ticket.

You can also through ticket on Aer Lingus via Dublin, and Alaskan to Portland OR then BA.

Can't you get a through ticketed open jaw?

https://www.heathrow.com/connecting-flights

Once you have an idea of which flights you want plug them into the connections tool o; the Heathrow website. With check bags you would have to go through passport control then to baggage claim. Get your bags and head to the baggage drop desk of the second carrier. That could be in a different terminal depending on the carriers involved. Then a security screening. Having only carry on bags and boardings cards would allow you to connect airside. I any case I would not attempt this without at least 4 hours between flights. Too many things that could go wrong to delay your arrival.

I have a similar issue regarding taking the train In my case the train from Kings Cross to York. Unfortunately giving a healthy margin for delays I would be on the train about 2 or 3pm. From previous experience that’s exactly the time I start to get tired and drowsy on arrival day. I’d be afraid of falling asleep on the train and missing my stop as the trains do not terminate at York. So I elected to do an overnight stay in London.

Many people find Heathrow big and cumbersome. If you can navigate a large multi terminal airport like San Francisco or NY JFK I think you’ll be fine at Heathrow.

You're getting great information but you didn't tell us when you are flying. There are seasonal flights from the US to Manchester but without knowing when your trip will take place, it's hard to give advice.

As an example, Virgin Atlantic flies from JFK in NY nonstop to Manchester. You could book the whole flight (SFO-JFK-MAN) on Delta. (Delta plane to JFK, VA plane to MAN with Delta codeshare.) This way you would bypass LHR.

Aer Lingus also flies nonstop from JFK to MAN without connecting in Dublin. You could fly AA to JFK and then Aer Lingus to Manchester. (Both are in the same alliance and can be bought as one ticket. )

If you don't already have your tickets, look for a flight on British Airways (SFO-LHR-MAN). This can be booked via either British Airways or American Airlines who codeshares these flights. If you do that, your bags will be automatically transferred from one flight to another. You would be able to use the Flight Connections passport control area rather than the main one. You would have to go through security again.

When you decide which flights you want to take, go to:

http://www.heathrow.com/flight-connections

to get step by step instructions on how to make the connection.

I flew Aer Lingus to Manchester last year (from Washington DC) and it went very well. We’re doing it again this spring. Connecting in Dublin is much less stressful than Heathrow, or having to take a train or bus to Manchester from London. On the return trip there’s the benefit of going through US immigration and customs in Dublin so you don’t have to do that when arriving in US. They usually have pretty good fares. Flight times a bit shorter too since US to Manchester via Dublin is geographically more direct than via London.

One other thing about going through Dublin is that there is a common travel area between Ireland and the UK so the Irish immigration you go through, which is normally pretty quick, is good for the UK too so you won’t have to do that again in Manchester. But whichever way you go you will need the new advance UK travel authorization.

The other thing to consider is the train or bus will get you to downtown Manchester. The international airport is out of town. You can take a train into town with involves walking to the station, paying for a ticket and then the time on the train. https://www.manchesterairport.co.uk/getting-to-and-from/by-train/