Fastest way from Heathrow to City airport

This www.londoncityairport.com/visitingtheairport/page/betweenairports should help.

Be anxious, be very anxious. This involves crossing from one side of one of the largest cities in the world to the opposite side, with no direct route.

• Time to get off plane, get through immigration and emerge at arrivals: 1 hour after touchdown.
• Time between airports: Heathrow Express to Paddington (fast but expensive) + tube + DLR to City airport = 1 hour 30. This info is from: https://tfl.gov.uk/ Going by road (taxi) will be a lot lot slower.
• Check in at London City = 30 Minutes.

Total = 3 hours, on a good day, and assuming you don't get lost finding the various trains, and you land on time.
Me, I would never suggest this. Has the person from the airline who described this as "doable" ever used either of these airports?

and did they guarantee what they would do if the plan fails?

Helicopter charter is your only feasible option. And I'm not even being facetious.

One potential saving grace - you only need to complete check in by 20 minutes prior to your flight at LCY, technically at least. Very ambitious though without an EEA-Swiss passport or with checked bags. Incidentally BA recommends 3.5 hours min. Not clear if the connecting flights are booked together or separately; in the former case the airline picks up the mess should things go wrong.

I wouldn't bother with HEX as it gives you a minimal time advantage with the possibility of having to wait 15 minutes if you just miss one.

Journey is 1 hour 20 minutes by Piccadilly Line to Green Park, Jubilee Line to Canning Town and DLR to London City Airport.

Thanks all. Yes I am more anxious now. But my calm and collected husband has to handle it. So that should make it better!

I am anxious if my connection time within Heathrow is less than 3 hours!!

This sounds very difficult if not impossible.

Soumyatha, what happened?

It looked from your other posts that your trip may have been in July. So did it work out for your husband?