There's a long thread in the BA forum about this that will likely answer your question definitively or get an answer from one of the Brits.

http://www.flyertalk.com/forum/briti...61-2004-a.html

I think the €300 is correct for the amount of delay you had. Your decision whether the cash or voucher is better for you.

See post #3 question #2 in the thread above.

Welcome to FT!