I've had two different stories from two different PO's regarding 1st Class recorded......
One says that records can't be classed as letters, even though it fits through the Large Letters slot, therefore must be sent as a Packet and charged £2.37. The other says no problem, and classes them as Large Letters (£1.65). The only reference to this that I can find anywhere on the Royal Mail website quotes, gifts such as books, etc, should be sent as Packets but no specific mention of records.
The last batch of records I sent went as a mixture of the two, but the packets only went as packets because they didn't fit through the Large Letters slot.
As you might have guessed, I will be using the 2nd PO for now, until I get definite confirmation that records are not valid letters.