I used to think it was some smart arse "trendy" new age phonetic teaching "aid", but now having researched it a bit it appears that you are correct and specifically we have, it appears, Irish Catholic nuns to blame for it.
BUT WHY, has it crept so much into England? (It appears to be quite common in Scotland, but am not sure about Wales)
Whatever, I don't like it, and why that letter? You wouldn't say Mem for M or Lel for L or Fef for F............