I'm worried by your last comment, Unclued.
6D entry begins 5 due to value of O, and ends 4 due to only one possible 3-digit value to power O. Then I wanted to find values of Z and U. At that point I have only values 1 and 4 unallocated from 1-6. Z +10U only works for U=4.
I agree with your value for I, but I've somehow got K=20, and M either 13 or 23. This hung together quite well, since I was able to find entries for 27A and 23D which don't clash...
Could do with confirmation as to which of us has the correct values for some of these letters really.