I have also booked using BA Avios AY HEL-LPA flights next Feb and started to wonder the same thing. Only paying the taxes, the flights did not cost too much (50000 Avios + €220.46 for two flyers).

Was able to select seats while making the reservation using BA engine. Hopefully there could be seat blocking as I chose seats 2A and 2C. Went then to change the BA FF number to AY one and ordered warm meals to both directions for €48. The other flyer is BA silver. Interesting to see, if have to pay for water...