Just in good nick. Not ripped to bits or looking really old. The booths want to exchange your money so they are unlikely to be silly about it. Also there are banks in Boro you can use. The one in The Rila is great.
But as others have said, as long as they are in fairly decent condition you don;t need to worry. At worst they may reject a tatty note, but I would think most of us will have a debit card to use as back up if we
a) can't exchange our £20 notes
b) if we need more spending money !