Fastest through each intermediate split, the Welshman clocked a winning time of 48 minutes and 54 seconds across the three laps which formed a 39.7-kilometre course in Northumberland.
The Team Sky rider came home 37 seconds ahead of nearest challenger Harry Tanfield (Canyon Eisberg), with five-time nationals winner Alex Dowsett (Katusha-Alpecin) rounding out the podium, 54 seconds back.
“It’s great. Obviously winning the Dauphine was a big boost. I’ve done a bit of training now since, and to come here and win the nationals for the first time is really encouraging. I rarely get to race in the UK, and to do a one-off TT like that is rare too, so I’m really happy.
“I was confident but I didn’t know what to expect. As I say I rarely do a one-off TT, it’s always part of a stage race where you’ve got tired legs and things. Obviously (Harry) Tanfield has been riding well this year and I’ve been watching him, and Dowsett it’s what he does. TT’ing in the UK is a strong discipline anyway, so to come and win is super nice.”