Ok no answer from email so I called them. Here's their answer:

1st bag is free, up to 23kg

Second bag is 15 EUR/kg per leg, and 130 USD.

So in my routing, there are 3 legs, so that comes to 45 EUR/kg + 130 USD.

No special preference if EBG (note SK925 is being sold as a BT flight number). I didn't think to ask about Baltic Miles as I don't have status on them.

So even thought BT is a lot cheaper than the other options for the ticket part, when I add in luggage costs, I'm better off paying more and flying *A and taking advantage of the extra free luggage allowance as an EBG.