The UK call centres open in the next 20 minutes, so call the UK number: +44 203 250 0145
ba.com/contact-us

It's best to do your own research here first, come up with a list of options, rank them and then as the agent to see if they can be booked. You can use AA, AS, Virgin, Delta and United, BA and AA strongly preferred but you can use other airlines in this scenario.

Have a look at these two resources:
https://www.flyertalk.com/forum/brit...need-know.html

and this PDF.
ba.com/disruption

Also once travel is completed have a look elsewhere in this forum about EC261 compensation.