Ryanair is £15 one way, BA just £3 cheaper at £12 one way for a bag. Those fares are optional indeed, but so is buying a bag on FR. Just because you select the options on a different page and it is worded slightly different does not mean you are not paying for a bag on BA.
Granted the second bag dimensions are bigger but BA still only allow one IATA bag plus one small 'personal item'.
Good luck getting those extra leg room seats too! Either you pay for them in advance (if you don't have status) or risk not getting them 24 hours before departure when it is free. FR is £11 for an exit seat guaranteed at point of ticket purchase. BA charge £8 for a standard seat, can't see the price for an exit row.