I'm looking for a place where I can pump large lugworm.

You can get them at Rhyl at low water, but you need bigger tides to get
the blacks, the yellow/green worms you're getting are blow lug,still a good
bait.

If you can't get big long worms but still want a big juicy bait try tieing two or three worms together side by side with bait elastic to make one fat juicy worm