Caerrnarfon
From Fenland, route between East Midlands and Birmingham, then via SWB VOR (and a LARS) then Hawarden (pronounced "Harden"), then a service from Liverpool and fly up the coast to Llandudno, then direct down the Menai straights with a LARS from Valley.
Alternatively, fly over Snowdonia, but get a LARS from Valley as the fast jets will be flying between the mountains.
Avoid Manchester. They are great and friendly, but it's a long way around....!
Going over the mountains is just wonderful if the cloudbase allows. Perhaps call into Welshpool en route.
Whichever way you go, the flying should be good.