The Dragons' territorial game plan and determined defence restricted the Exiles to just three penalties from Steven Shingler and a solitary penalty from Tom Homer that will give a lot of hope to Darren Edwards ahead of the RaboDirect PRO12 curtain raiser against Zebre on August 31.
Meanwhile, the Ospreys winless pre-season continued as they were beaten 21-10 by Bath at the Liberty Stadium.
The West Country side produced a storming first-half display to lead 18-0 going into the break with former Leicester Tigers fly-half Sam Vesty kicking eight points and former England internationals Dan Hipkiss and Lee Mears each scoring a try.
Replacement fly-half Olly Barkley and Dan Biggar exchanged penalties as the home side still had a mountain to climb.
Towards the end there were encouraging signs to take from the game for the RaboDirect Pro12 champions as winger Tom Grabham crossed after a well-worked passage of play with Biggar adding the extras.
The Scarlets' first pre-season outing ended in defeat as the Worcester Warriors ran in three tries to secure a 22-7 victory at Sixways.
A traffic jam delayed the kick-off for 35 minutes, and there was an equally slow start from the visitors.
New Warriors signing Josh Matavesi impressed as the former Racing Metro centre bagged two tries in the game.
A further try from winger Ben Howard and seven points from the boot of Joe Carlisle secured a convincing victory for the Warriors.
Scarlets centre Nick Reynolds scored a consolation try which Dan Newton converted.
Elsewhere, an inexperienced Cardiff Blues team produced a resolute performance as they went down 20-8 to Gloucester at Kingsholm.
Gloucester took a 10-0 lead going into half-time with new signing Billy Twelvetrees contributing all the points after his mazy run saw him score the first try of the game.
A Gareth Davies penalty got the visitors on the scoreboard early in the second half before Welsh wing sensation Harry Robinson crashed over after neat handling between forwards and backs.
Another Tweletrees penalty increased the Aviva Premiership side's lead before the Cherry and Whites sealed the victory with a penalty try given for repeated 5m scrum infringements against an over-powered Blues pack. Twelvetrees added to his points tally with the straight-forward conversion.