As rcspeirs says, you won't find much expertise on BritRail passes among Brits. It sounds expensive for what you want to do, given the price of a flexible London-Liverpool return quoted above (£82/ $102) and the cost of a single Heathrow Express (no more than £22 booked online, as little as £5.50 inflexible booked in advance) or Tube (£3-£6).

The benefit is the pass covers you for both journeys, without worrying about tickets or validity or peak times or committing to a single train, but for me it's a hell of a price to pay for that flexibility.

That London-Liverpool ticket can't be used ex-London between 1440 and 1907. If can travel outside those times, then you could just get the tube or HEX + tube to Euston and buy a ticket at the ticket office and it would be cheaper than buying the Brit Rail pass. Better still, the option of a flexible outward portion + inflexible inward portion would be cheaper.

If you want to travel in the evening peak, and don't want to commit to a single train, then the London-Liverpool ticket rises considerably, so the Brit Rail pass looks like good value. (I assume the pass is valid on peak-time journeys - we'd need to look into this!)

Phew, it's not easy, negotiating UK rail fares. Are you sure you can't fly into Manchester? ;-)