Heathrow to St. Pancras

I usually give myself about 1- 1.5 hours to get through Heathrow after arrival. Most of that time is spent walking around the terminal or waiting for luggage. Passport Control itself is very quick with e-gates.

Depending on when the next train arrives, Heathrow Express to Paddington and then Circle or Hammersmith and City Underground lines to St Pancras is one option that could be slightly faster. It will be pricier. The other option is simply to board the Piccadilly line at the airport and take it directly to St Pancras. Listed times average about 60-70 minutes for the journey at a cost of 5.80 GBP.

If everything has gone smoothly at the airport, I would choose Piccadilly line. This eliminates any needs for transfers during the trip.

First, take Heathrow Express (HEx) train from Heathrow to Paddington station (takes just 15 minutes, and you can get early bird discounted tickets on the HEx Website), then second, take the Tube from Paddington to St. Pancras.

I just take the Piccadilly line direct. Less hassle.

Piccadilly Line. Takes about 56 minutes, no changes. Pay with contactless.

There really is no point to me in taking the HEX and changing rather than the direct Piccadilly tube. Having to change means that you lose any time saving, you have a change and you’re paying more.

If the main concern is for the fastest route, then it is Heathrow Express to Paddington (15 mins) + about 5 mins walk + Circle/Hammersmith and City line to St Pancras (10 mins) = about half an hour plus any waiting time. The Piccadilly line is over an hour.

The only situation where I would take the Piccadilly line to/from Heathrow is if I was coming from Hammersmith or somewhere further west.

would not the Elizabeth Line to Faringdon and then easy transfer to Tube or Thameslink one stop to Kings Cross St Pancras (tube) or St Pancras (Thameslink) be fastest of all, even including the Elizabeth Line stops?

Less frequent than the tube but so is the HEx.

The Thameslink station is just under the main concourse at St Pancras and has lifts. Then a short walk to Eurostar check in.

Nigel, that was my first thought as well but looks like that is closer to 40 mins + waiting time. But for me, that would be the most convenient route.

Euston to Heathrow Central via Tube/Elizabeth Line is 44 minutes, knowing exactly where I am going, and no wait for the tube or the Elizabeth Line. St Pancras will be about 3 minutes longer. Coming from Heathrow it will be longer if you have any wait for a train.
With no waits Heathrow Express/tube should be about 10 minutes faster, allowing for a very adequate 10 minute transfer time at Paddington.

Time is more important than money here. You need to be at St Pancras by 1145. Say an hour by whatever method used, you need to be leaving Heathrow by 1045. That gives little wriggle room for a late flight, immigration delays etc.

The first thing to do while in the immigration line is to check the apps for any delays. There is no point wasting time going for the HEX or Elizabeth Line if they are having a bad day, likewise for the tube. Then if everything is working well find out on the walk to the station which is the next train out - HEX or Elizabeth Line and take that. The difference in fare is only about £14.
Not a Kings Ransom, this is not the time for penny pinching.