I did my marine biology degree at Aberdeen, and collecting on the east coast was rubbish, all of it was done on the west coast. Scotland is odd like that. But that said, provided you stay away from harbours and sewage outfall pipes, you should be fine. Stuff washed up on the beach especially will have been scoured by the sand and bleached under the sunshine, so while it might look a bit beat up, it should be fine. I used to collect rocks and stuff for my tanks like this all the time on Aberdeen beach. Even put in things like seabird skulls (they look cool, but disintegrate after a while). Ah, happy days!
And if you can get a bit further north, around Inverness and Cromarty, the water should be spotless there. We used to find all kinds of neat sea creatures, not to mention the odd porpoise bone.
Cockle shells should be fine. Not really estuarine, but they'll substitute nicely for other clams. One thing I'd mention is to avoid putting just perfect shells in the tank. Smash some of them up first. Looks much more realistic. Look around a real beach, and see how many perfect shells you can spot!
Cheers, Neale