Just to round up this thread, we have now reached England on our way back. We deciced on Hook van Holland with a night crossing to Harwich. Booked a Flexi ticket early since it could be cancelled at no cost until 24 hours before departure. Very nice and comfortable ferry, a good trip.
Then Holyhead - Dublin, also booked ahead. You do need quite a lot of time to cross England, since a lot of the roads are both small and congested. Uneventful 2 hour crossing at a boring ferry.
On the way back it was Larne - Cairnryan. Booked 2 hours before departure, no problem. Relaxed 2 hour crossing. We have just booked Harwich - Hook van Holland for the way back.
We enjoyed both Western Mayo and the Antrim Coast Road. Especially Torr Head and Glens of Antrim was nice drives. Thank you for all the good advice.