Thanks for the responses, I was not delayed at destination, I had to actually take a flight the day prior from an alternate airport to reach LGW where my car was parked.

In terms on when I asked for cancellation/refund of the cancelled BA flight, this is the sequence of events in detail:
- BA informed flight (October 8) was cancelled on 5th October early morning via email
- Customer calls BA customer service over phone in the morning 5th October, asked if possible to rebook in IAG flight from different airport GRX-LGW on October 7th as most suitable option reaching LGW and not been late to work on Monday, despite been a day prior. BA answers NO
- Concerned about the possibility of not getting the desired flight and tight timeline, I booked the Vueling flight GRX-LGW for October 7th for entire family on October 5th
- On October 7th, while waiting at airport to board on GRX-LGW flight, I cancelled for a refund the other booking to get my money and points back. So I did cancelled before the original flight was due to take off, I honestly had no idea this could have any impact on my ability to get refund or compensation so a learning process for me.

One side note is that the flight BA had cancelled was booked with BA points+money+ use of my BA companion voucher (AMEX reward) - when I got refunded, I had something like 1-2 month left to use that companion voucher and it obviously expired without using it as I was not travelling again in that period.

I hope the timeline above helps and appreciate any guidance if possible to claim the cost of alternative flight, the difference between the alternative flight and the original BA Flight in cost (not an easy one as there were BA points and companion voucher involved, not sure how to price) and if anything at all related to companion voucher expiring.

Thanks,
paninovic