The point remains that BA were obliged to rebook you at the earliest opportunity - including on other carriers or means of transport if appropriate. It's impossible to determinitively say whether there would have been seats available on SN/Eurostar, but it seems unlikely that there would have been no seats at all.

If BA failed to do apply all reasonable measures to mitigate the delay, then it's irrelevant whether the actual reason for the cancellation was outside their control - you are entitled to the compensation.

Whilst CEDR might still apply that limb of the UK261 exceptional circumstances test without you having raised it, there's no guarantee they will. I suppose you will just have to hope that they do.