I would follow the guidelines on the BA website.

Thanks for letting me know - I will pass your message on.

In addition to your hand luggage, you can take a laptop or a handbag onboard.

http://www.britishairways.com/travel/bagcabin/public/en_gb 

Kind regards

Olly, your editor

I am booked on Treasures of Tuscany, flight with BA. I see that the hand luggage details you have sent do not agree with the ones on BA's website.

You letter says 1 piece 45 x 20 x 36 cm, but the website says one piece 56 x 25 x 45 cm plus handbag or laptop of size you quote.

The bag I was planning to bring is 22cm wide, so I need to know as the other bags I have are considerably smaller. Alternative is smaller bag plus handbag, is that acceptable?

Regards