Gatwick or Heathrow?

Aside from the cost of flights, Gatwick is a smaller airport and therefore somewhat easier to get around than Heathrow. As far as the time you quoted, I suspect it will be longer than that. Are you planning to go directly to Manchester after arriving in London? While 40 minutes will get you to London Victoria, that may not be where your train to Manchester is departing from so you'll need to add time on the Tube or whatever. The train to Manchester may depart from London Euston or one of the other stations. I'm sure Nigel or one of the U.K. members of the forum will be able to provide more specific information on that.

Of course, the easiest solution would be to simply fly into Manchester. If you can't find cheap flights from SEA to MAN, have a look at flights from Vancouver with Air Transat.

I've heard that Seattle to Gatwick can be significantly cheaper than
Seattle to Heathrow

There are very few transatlantic flights that fly between the US and Gatwick. 99.9% use Heathrow. That will change next year when Norwegian begins some flights, but I don't believe Seattle is in the mix.

Manchester train leaves from Euston.
We want to travel via London because our trip will end there with a weeks holiday.

I've flown into Gatwick from the US several times and honestly I haven't found it any less convenient than Heathrow. Either airport you'll have to travel into the city with at least one change to get to Euston.

DJ

"We want to travel via London because our trip will end there with a weeks holiday."

You can fly IN to Manchester and OUT of London. This kind of ticket is called Open Jaw, as you probably know. It keeps you from having to do just what you are talking about doing, with having to take the train to Manchester. You don't have to enter and leave from the same airport. You may want to consider this even if you have to buy two separate tickets, one over, and one back. Just more convenient flying into Manchester if that's where you will meet the rest of your party.

Fifteen years ago, the direct British Airways flights between Denver, Colorado and London landed at Gatwick. Flying onward to most anywhere else involved a transfer by bus to Heathrow. Now the Denver>London flights land at Heathrow.

Gatwick was a convenient, smaller facility than Heathrow (but still big), and the Gatwick Express train got us from the airport directly to Victoria Station in London - really handy so long ago. That would be excellent if you could continue onward to Manchester from Victoria Station, especially if the price and/or travel time worked for you.

Now there's also a Heathrow Express train, but we haven't used it except for getting between terminals at Heathrow.

If you follow Rebecca's excellent advice, you want to look for "Multi City" (not "One Way" or "Round Trip") on booking sites such as the airlines' own websites, Google Flights, Kayak, etc. You can use LON to search for "all London airports."

It may not be any more expensive to fly SEA-MAN and LON-SEA than for a roundtrip SEA-LON. But even if it is, you save a lot of time, money, and hassle doing this, so it's worth a bit extra.

What are your options for Seattle to Gatwick? The direct flights from Seattle (BA and Delta) go to Heathrow. As far as I know, getting to Gatwick would involve a stop somewhere else to change planes. Do you really want to do that?

If you can find flights into Manchester from a US gateway, that might work, but you will have to make a plane change somewhere. So your choice may be a direct flight from Seattle to Heathrow plus train to Manchester, or a two-leg flight to Manchester. Please let us know which is more cost and time effective and which you choose.

Nobody's explicitly said it so far, but it's also worth looking into domestic flights from Gatwick or Heathrow to Manchester Airport. The fares may well be competitive with rail if you want a flexible ticket in case of flight delays.

Unless you want to enjoy the view from the train going from London to Manchester I think your best solution is to fly to Manchester, and the take a train to London (and enjoy the view here), and go home from there.

http://www.momondo.com can find some very good options for this. I found return trips for around 1.630 USD/person, but not knowing your specific dates I don't know if they work for you.

If you want more options try http://www.rome2rio.com

I think the Icelandair connections from Seattle to Manchester and then from Gatwick to Seattle are the best solution. However, since you are in Seattle, you should also consider direct flights from the Vancouver BC airport to Gatwick. Look at prices on Airtransat and WestJet--they are remarkably cheap and for a bonus, they are in Canadian dollars.

I got a great deal to Gatwick. If you go to Gatwick , bypass the expensive Gatwick Express and book a Thameslink train to Farringdon or City Thameslink station for much less.

Ditto on the Icelandair suggestions. The connection through Keflavik is very easy with short time on the ground. I know that Icelandair flies into Gatwick but I have only flown them into Heathrow and had a great experience. I have used both Gatwick and Heathrow as arrival points in London but I just prefer Heathrow.