Putting a blanket policy in place would make little sense for BA (why give passengers flexibility on routes that are not being significantly disrupted?). The weather for Heathrow isn't forecast to be too bad. Wind is forecast to be pretty much straight down the runway for much of the day, so limited crosswind component. Some stronger gusts. Arrivals are regulated so there will be some delays and likely some cancellations, but BA will probably target cancellations on routes where they can accommodate disrupted passengers on alternative flights.