Will you be allowed to enter the U.K if you were convicted of a felony?

Question

I am getting married at the end of august to a uk citizen, and then moving to live in the uk. I already have my visa and know ill be allowed to enter, but since we are getting married here, my fiancee and I are having a big reception in Scotland for all of his friends and family, and my mother and father are supposed to come over for a week so they can visit with his family and go to the reception. The big problem is my father was convicted of a felony in 2000. It was not a violent crime ( he was busted for growing 2 pot plants) he served like 6 months in jail and was released. Since then he has been clean and not had any more problems. He applied for and was given a passport last year with no trouble. Yesterday he was trying to travel into Canada for a sailing competition, and was refused entry. SO now everyone is very stressed out and worried about if he will be allowed into the U.K or not.

Answer 1

did the people in canada give a reason for refusing him entry? As far as I knew convicted felons were not allowed to leave the country, but there may be a time limit, like the 7 year time limit on job applications