Good question. Here's the note on R41:
"AWY base is FL 35 or 3014 ft amsl, whichever is higher. Consequently, when low atmospheric pressure prevails, FL 40 may not be available as a cruising FL."
BTW there's some discrepancy in international usage of the phrase "transition level". In some places it means the lowest level with 1000 ft of separation from the transition altitude. In the UK, it's simply the lowest multiple of 5 flight level above the TA. (E.g. QNH 1014, transition level is FL30).