London to the end of the M54 you might as well stick to the motorways and get it over and done with, from the end of the M54 keep following the signs to Llangollen, series of roundabouts joined by dual carriageways then it will be roundabouts joined by single carriageway for a few miles, then at the far end of the next Dual Carraigeway turn left to Knockin, keep on that road till you see a sign for Bala pointing left, TL, up to T, TR to Bala, SO at Bala to Trawnsfynedd TR at T to Porthmadog, that leaves you to decide where you are going in Gwynedd
Shep
Lots of other ways but that's my pretty route but still quick and quiet.