If BA reroute you on to another airline, then you get the full miles and tier points, as per the BAEC rules.

I think last winter there were also miles and tier points if BA cancelled your flight, but I think that was a concession rather than part of the rules. I am not sure what happened if you cancelled a flight that was expected to operate.

I suspect at the moment miles and tier points are the least of people's worries. I have 12 sectors coming up and missing the first will be farly disastrous, so I'm hoping normality returns by Wednesday