Grey sheets come out once a week and give buy and sell number at given grades. As Oz stated knowing how to grade comes into play. Think of the
Red Book as the window sticker of a car, few walk in an just say WOW great price I will take 2! Many dealers "not all but many" over grade there Raw coins and will not let you know that a given coin is a problem coin. Like it's been cleaned or that the toning you see is artificial, or that scratch and any one of this type problem will preclude it from being graded by a top tier grading company like PCGS, NGC, ANACS
The best advice I can give is if you are looking at
Morgan dollars and you have a base line understanding of what an AU and BU coin is. Go out and buy 10 or so common date coins they should be around 25 to 35 dollars, Your in England so call it 20 Quid. Once a day post a few photos of just one coin here. We will let you know what we think and if you do make an error, with silver at 28.00 + US the worst you can lose is 5 Quid and and good lesson is worth that. On the good side many folks on your side of the pond know little about Vams and you may have better picking over there and have a better up side.
My niece is at Lakenheath over in your land, things over there are cheaper compared to the same type antique sold here. Its just a little different market, find the gap between the 2 and you have a good investment