It sounds like airlines are being asked to reduce flights (assume: passenger numbers) by 30% during the strike.

As seems to be the norm during this kind of disruption, focus ‘should’ be on cancellation of short-haul over long-haul, although there will inevitably be disruption and can’t imagine any airline will be unaffected.

For rail strikes, Heathrow Express is only impacted through limited service hours early mornings and in evenings. If you are planning to travel during daytime hours to central London there shouldn’t be much, if any, impact. I believe TFL services are also running as normal.