Originally Posted by
Flying Lawyer
I understand it either as a cancellation or as a denied boarding situation and they were under the obligation to book me on the first alternative flight, not on the first available BA flight.
Your understanding is correct, and you should have insisted on an immediate escalation and going over the terms of their Tariffs, IATA Resolution 735D, and the BA and IATA guides on re-routings to "refresh their memory".
Originally Posted by
Flying Lawyer
You are right and I did not take a "no" for an answer. However: The only thing they were good in was ignoring my argument.

I am afraid they won:
Your damages, by your own admission, amounted to 300 GBP. BA offered to reimburse you the full amount. BA was trying to avoid paying the cost of re-routing and they succeeded.
In contrast, if you had obtained written documentation of their refusal to re-route you on a first available flight (of any IATA member carrier) and were forced to purchase a replacement ticket, you could have now made an argument that BA violated its Tariffs and claimed the cost of that ticket on LH/SQ/etc...