Try (A↔Z, B↔Y, etc.):
t → s h → g m → l y → x l → k
So maybe not Welsh plaintext. thmyl — could be ‘the mill’? t h m y l → remove h, thmyl → ‘themyl’? No. If th = voiced th (as in ‘the’), m y l = ‘meal’? ‘the meal’? But missing e. thmyl lbt jyms bwnd llandrwyd mn mydya fayr
Better: Try (common in puzzles):
lbt — ‘lbt’ = ‘lob it’? unlikely. jyms — ‘jyms’ = ‘gyms’? (j=g?). bwnd — ‘bwnd’ = ‘beyond’? (bwnd → b w n d, add e o? ‘beyond’ has 6 letters). Actually, let’s test Caesar cipher with shift of +1 (a→b) but backwards? No, systematic: Try (A↔Z, B↔Y, etc