Connection from London City Airport to Heathrow for flight to US

DEN = Denver, USA (I had to look it up).

Going out looks OK, both flights are Heathrow. 4 hours is plenty.
"I'd have four hours at Heathrow to land, deal with the formalities," - there should be no "formalities", just security. You will transfer T3 to T5 "Airside", never legally enter the UK and your bags should be checked through.

Coming back, getting from London City to Heathrow, my initial reaction is "no way". London City is the opposite side of London to Heathrow. You will need to go straight through the centre, and at the tail of the morning rush-hour. But you do have 6h50 to do it.
And you will need to go through immigration at London City to enter the UK, and exit again at Heathrow.
If you are willing to do this "London experience", why not?

Have you ever been to London City? It is a small short.haul airport built in the old docks. See photo here: https://www.passengerterminaltoday.com/wp-content/uploads/2019/01/web_London-City.jpg
It is connected to the rest of London by the DLR - a sort of automated (driverless) elevated railway.

I think 1 hour is to short, I would estimate 1h30.
However I would take the tube all the way, that is fewer changes:

From: London City Airport
DLR to Canning Town DLR Station, Journey time: 6 min
Jubilee line to Green Park, Journey time: 18 min
Piccadilly line to Heathrow Terminals 2 & 3, Journey time: 46 min

Thanks, Chris. Sorry about the unexplained abbreviation (I'll fix it if I can).

This will be I think my 10th flight (at least) into London, but those flights have all been into/out of Heathrow or Gatwick. I've never used London City airport. On all my prior trips into London, I've stayed in the UK, mostly in London. I'm fairly familiar with the TfL system, and have taken both the Tube and Heathrow Express to/from Heathrow in the past. Still have ££ on my Oyster Card from my trip last year.

Thanks for the tip about the Tube route. When I posted above, I'd searched the TfL site for the fastest routing and it sent me to the Heathrow Express. I later searched for fewest changes, and then saw the Tube-to-Heathrow options. You are correct; those journeys are estimated at about 1 hour 30 minutes. I think that if I can get from LCY to LHR in less than three hours, I should be fine for this connection.

Is the TXL to LHR flight much difference in price?

The return journey should work, but how does it work if you are delayed (broken tube train, or something like that) and miss your flight. Will you be put on the next available flight or will you have to buy a new ticket?

Personally, I would gladly pay extra to not have to travel between airports in London.

(By the way, it's common in Europe to refer to airports by name and not their three letter codes.)

I have flown in and out of LCY numerous times and have had to get to Earl's Court. Never more than about one hour then another 45 minutes to Heathrow.

Check in for both flights in Berlin then all you have to do is a bag drop at Heathrow.

LCY is very small and you should be out quickly.

Follow Emma"s advice regarding change stations as it will be easier with luggage. And avoid Green Park like the plague.

Thanks to all of you. Very helpful information and advice!

The fare I've placed a hold on, is at least $300 (£245) less than any fare I've found for a reasonable - or even unreasonable- round trip ticket between here and Berlin. ("Unreasonable" meaning crazy routing, extra stops, scary-short same-airport connections, and total travel times that would make me exhausted when finally arriving in Berlin. And perhaps in pain from hours in economy seating. It's happened.) And, this fare is for premium economy seating on the Denver-London legs of the flight - at a price that would be a good deal for economy.

I have another 48 hours to decide whether to book these flights.

Having spent a fair amount of time in London over the years, I've experienced a Tube strike that shut down the only line serving the area around my hotel - on a rainy day when I had to get from Bayswater to Victoria Station to catch a train - and have several times been on a Tube platform when it was impossible to fit more bodies into the train that arrived so we had to wait for the next one. I've ridden crowded TfL buses, squished into closer proximity with other passengers than any of us would have preferred, sat on TfL buses stymied by London traffic and nowhere near making the schedule, had to re-route journeys on the fly when a Tube line was shut down unexpectedly, and had excellent cab rides and a bad one from a (I think unlicensed) guy my "helpful" hotel staff called for me. I've used the Tube to travel between Heathrow and Central London, as well as the Heathrow Express. I've walked several blocks with my luggage. And, I've coped. Absent these bits of experience with London and its transit options, I'd be very leery of this change-of-airport connection.

But with this history, and a little help from my online friends, I might get by with this one. If I decide against booking the flights it will likely be for other reasons (personal and family circumstances) than the airport connections for the flight home.

Thanks again.

In normal circumstances, I think you’ll make it. The main thing I’d keep an eye on is the climate change protesters. Shutting down the tube on a weekday rush hour is something they very well might do.

Suz, I think you are absolutely right to take your extensive London experience into account when considering this. That, and the time you are taking to plan ahead, make me think you could make this work.

Do be aware that London City is more susceptible to disruption from weather issues because of the short runway and steep approaches. I know someone who was stranded in Luxembourg from yesterday overnight because of thunderstorms in London. That is impossible to predict of course. Morning fog is a more regular occurrence but by May not so much.

Thanks, Marco. That's something to consider since it's an early morning flight.

I'm glad that BA allows one to hold a fare for 72 hours for a $10 fee. It's giving me time to get over the excitement of the low fare and explore the practicalities. TBH I'm leaning against booking that itinerary. If I do book, BA will credit the ten bucks against the fare, and if I don't book, I'm out ten bucks. A small price to pay, in my opinion, to avoid a costly booking mistake. You can't even buy a decent guidebook for ten bucks. (I've been borrowing guidebooks to Germany and Berlin from the public library, and putting the best of the lot on my Amazon wish list.)

This experience has shown that I really hate the idea of using Heathrow merely as a connecting airport on a "city break" kind of trip, without setting foot in London. I've also been working on itineraries combining a stay in London and a stay in Berlin. It's back to the "drawing board," or in this case the flight search sites.