Most of the time, the quality of the bait you buy from Fishing tackle shops is only as good as the bait suplier. Personaly I have never had any problem with fresh lugworm bought from Lewis Tackle in Whitley Bay.
When you ring them up and enquire about worm availability, they will always let you know as to the quality. For example : " Yes we have lugworm in, but the worms supplied to us today are quite small mind "
Have to point out that most of the time the lugworm from here are of a decent size, but the point I am making is ; if theyr'e not, then they will let you know. Even then if you decide to buy them they always put a few extra in each packet to make up for this.
Best thing to do mind is to buy a decent fork and dig your own whenever possible.