First point: Don't assume he'll automatically "get" it just because he's gay (or mostly gay)(and I get "mostly gay" just fine, my best friend is a gay man who was recently divorced form his wife of 13 years, having been openly gay the whole time).
Second point; Don't assume he automatically won't "get" it just because he's older, cis, or whatever.
Third point: You know him best. What's the best way for him to take in complicated information? Does he need time to process it or maybe research it on his own? If so, it might be best to tell him in writing. Does he need to discuss things and ask questions in order to understand them? If so, you probably want to tell him face-to-face, maybe over some father-child bonding substance like ice cream. Does he have the patience and attention span for a full-length explanation, or would he rather just hear a short 1-2 sentence statement and then ask questions if he doesn't understand? How much have you let him see of your girl-self? Would he have any memories of something in particular that would reinforce in his mind that this is a longstanding issue? Or will this seem sudden to him? If it's going to seem sudden, what can you do to reassure him that it's not a phase and that you've thought it through? How much does he know about trans issues and terminology?
Fourth point: Make sure he has someone else to talk to after you come out. You have your therapist. He needs someone too. Point him to PFLAG or the UK equivalent, or see if it's ok for him to call your therapist and talk, or find some other resource you can set him up with so he doesn't feel alone.
