Personally I wouldn't go with a flying gate, but that's just because I don't like them. At least you could lots of extra tubes to beef up the design without making the bike look any uglier.
I suspect one reason why Mercian didn;t want to do it is because they aren;t really super custom bike builders, more a small factory, and a bike for your dad will need some extra thought adn a different approach. Maybe a custom DT, using a DT for the TT and maybe two TTs to stiffen the main triangle. For blades for chain stays etc. I would go for Dave Yates because he is first and foremost a very skilled engineer who makes genuine custome bikes and so will be able to think this through and come up with the best design. It's also going to be almost impossible to make a frame for your dad using lugs because many of the tubes will have to be oversized, so you might want to bear that in mind.
Personally I wouldn't go with a flying gate, but that's just because I don't like them. At least you could lots of extra tubes to beef up the design without making the bike look any uglier.
I suspect one reason why Mercian didn;t want to do it is because they aren;t really super custom bike builders, more a small factory, and a bike for your dad will need some extra thought adn a different approach. Maybe a custom DT, using a DT for the TT and maybe two TTs to stiffen the main triangle. For blades for chain stays etc. I would go for Dave Yates because he is first and foremost a very skilled engineer who makes genuine custome bikes and so will be able to think this through and come up with the best design. It's also going to be almost impossible to make a frame for your dad using lugs because many of the tubes will have to be oversized, so you might want to bear that in mind.