Try (A↔Z, B↔Y, etc.):
t (20) → g (7) h (8) → u (21) m (13) → z (26) y (25) → l (12) l (12) → y (25) thmyl lbt jyms bwnd llandrwyd mn mydya fayr
Better: Try (common in puzzles):
t (20) → q h (8) → e m (13) → j y (25) → v l (12) → i Try (A↔Z, B↔Y, etc