Hmm and how do you want the result displayed? You can have a formula to convert 215 to 15st 5lb, but as the cell would then conatain a mixture of numbers and letters it would be difficult to use that result in further calculations. If you just want the weight displayed in st and lb then that is a simple formula.
If cell A1 = 215 then in cell A2 place =INT(A1/14)&"st "&ROUND(A1/14-(INT(A1/14)),2)*14&"lb"
Do you mean that you want to enter 215 into a cell and have it convert that value in the SAME cell to st lbs? If I am understanding that correctly if it could be done at all, I think that would require some VBA code which is beyond me, put others here might be able to help with that.