As I said in the CC forum, this is a tough one.
We are trained that statistically, your chances of being attacked when intervening in a domestic disturbance is very high. Both parties will turn on you. This is evident on the ground as it is in the air.
Separating them was a good idea. If it was my flight, I would have been pushing for restraining action at the point where he was dragging her back to her seat. Crew would have also had to prepare to restrain her, if need be.
Diversion would have been a last resort as it does inconvenience everyone and will cost money. You are also leaving the passengers in an outstation that will do nothing more than put them on the next flight once they've sobered up.
Arrest on arrival in the UK is probably the best option but as Tezzer said, they were taking pills. I can see the pax claiming stress, alcohol combined with pills, squeezing out a few tears and copping a fine.
Clearly, it is unacceptable. I know I would find it difficult NOT to get involved. I couldn't stand by and watch anyone being hit on the aircraft. However if you are a young, slim, female crew member, it would be very frightening and you have your own safety to consider. You can't always rely on other passengers to help you out, either, in situations like this.