elevators or escalators in tube stations

Even though I assume that both stations have those options, a taxi is easy and not a big splurge (maybe 15 pounds) for two people with luggage. Station details can be researched at https://tfl.gov.uk/transport-accessibility/wheelchair-access-and-avoiding-stairs.

Are you concerned about luggage, or a walking disability?

You can use this step-free guide to tube and train stations:

http://content.tfl.gov.uk/step-free-tube-guide-map.pdf

From the Heathrow Express platforms at Paddington there is direct access to the walkway to the Circle and H&C lines that go to St. Pancras, but I can't recall if that is steps or escalator. If steps, you can go around the other way, through the station and following signs to the tube. These lines are barely underground at Paddington. At St. Pancras we used the elevator with our luggage.

A lift has been installed to the platforms on the Hammersmith and City part of Paddington - this is situated at the far end of the platform to the main line station concourse at the Bishops Bridge Road entrance. The route to this involves walking up a slope. This is also the way to the taxi rank.

Kings Cross St Pancras underground station also has step free access from platform to street level.

Thanks for the information. A taxi seems to be the best option for four old ladies with luggage. A single ticket on the underground is quite expensive.

as long as the four old ladies don't have too much luggage or they may find that it is both difficult to fit the 4 old ladies and all the luggage in the taxi and onto the Eurostar.

Make sure that you are at St Pancras International with plenty of time as there are often queues and you need to be at the head of the queue and checked in 45 minutes before departure. They are strict. After check in you go through security and both British and French border control before getting to the departure lounge.

If you bought a through ticket from Bath to Paris as recommended to you on your other thread, then the underground would already be included and would not be 'expensive'.