The Basic fare passenger can add 1 bag for £40 if done online, or £65 at the airport. That is certainly for 23 kgs, but may be 32 kgs for the Gold cardholder (one of those things where experiences/opinions never seem to get a final landing). The second and subsequent bag(s) is/are £60 in advance, £75 at the airport.
Note that in the LHR-ATH direction you can usually check in bags for free so long as they conform to the hand luggage rules. That's instead of a hand luggage allowance, not as well as. You can't make that assumption for ATH-LHR, it won't work in that direction.