People normally check using tools such as ExpertFlyer, as KARFA has done for you. If you don’t have access to such tools, in future you can post flight details in the availability and flight load thread as linked below, and somebody will usually help you

Help to check BA seat availability and BA flight loads

Based on the availability posted above, although it’s half term and the flight could be heavily booked, it’s possible BA has only opened the highest booking classes in each cabin so far, and will open more later. I’d be keeping an eye on this, you may find D class opens in the coming weeks.