Important update

BA are now issuing eVouchers directly in simple cases. These are usable online. Complex cases still get FTVs, which require a phone call to book. In both cases, you need to apply online through the Cancellation Options in MMB, and both will generate an email typically within a few minutes. This is how to tell the difference

1) eVouchers will get an email entitled "Your British Airways eVoucher"
This will then have a line like this and the online ability is mentioned in the email text:
Your eVoucher details
125-4200412345 / GBP48.87 / WAGE-SLAVE /

2) FTVs will get an email entitled "Your British Airways Future Travel Voucher"
The relevant line then shows:
Voucher code(s)
125-2110754321

It doesn't take much, by FT standards, to turn a booking too complicated for the automated eVoucher. POUGs, flight changes, TCP, seat payment, pay payment with Avios, UuA. 48 and 72 hour Hold bookings all stop it. But if you made a simple single or return booking, point to point, on BA.com and didn't change it, then you should get an eVoucher.