Elizabeth Line (Crossrail) to Whitechapel and a 5 minute walk is your best bet - should be a direct train taking ~45 minutes.

If there's no Elizabeth Line train to Abbey Wood showing (that's the branch with Whitechapel on it), take a Shenfield train and change somewhere like Paddington. However, I think for the next 6 months at least, all the Heathrow trains are only going to Abbey Wood.

The Heathrow Express is quicker from Heathrow to Paddington, but terminates on the high-level platforms. By the time you'd made it down to the low-level Crossrail platforms you'll have lost any time savings from the quicker train!