There are two lounges accessible to CSA & SkyTeam fliers, one in the Schengen area, one in the non-Schengen area. I never used either as I have flown out of PRG as either an OW or a *A elite, so can't tell you for sure what the deal is as a SkyClub member. It is possible that the T2 lounge is the official CSA lounge (thus open to SkyClub members) while the T1 lounge is contract (it certainly displays a very long list of airlines which can use it). Delta pax would use the Crystal lounge when flying to NYC. I don't know that you will be allowed in the Crystal lounge if you have a SkyClub membership and are flying on BA, it's definitely not a SkyClub and access will be restricted to those flying on the airlines listed at the entrance of the lounge.

FWIW the PRG lounge has had a major cosmetic makeover this summer, likely due to the fact that it is the lounge in use for the Emirates flight. Food when waiting for the morning flight to LHR is still as scarce as described above, might be better later when the Dubai flight leaves. General look and feel of the lounge is much improved, though seating is a bit compressed and the whole place feels like you're a bit boxed in now as the seat backs are very high.