Rail pass-Heathrow express

Yes, they are valid and you can activate the passes at the Heathrow Express station. This is particularly a good value if you'll connect out of the city by train on the same day, e.g. from Paddington station to Bath. Of course, if you chose a consecutive-day pass, then you're not counting travel days in the same way.

In other words, you burn a whole day of pass usage for that one relatively inexpensive trip if you're not going further than London.

Thanks. Yes we are taking a connecting train to Bath.
Thanks for both remarks