I think the answer is the same as it is for buying any coin as investment/collecting, if there are millions of these things and if they are all the same such as a krugerrand. where the design stays the same and there are no particular rare years, then it has no collector value at all, its just a piece of bullion. The same is true of the Britannia's and Eagles, nothing much changes, however the Royal Mint have hit upon the idea of producing limited edition bullion coins. This means with the QB you have now 11 different designs to collect, you can't just go to the mint and buy any one you like as they have sold out of most of them, hence the secondary market and people prepared to pay just a little more for say The Lion of England etc to complete the rather nice set.
The situation with the CP's is that they feature a cuddly Panda, the design changes each year and they appear as proof like, again these features make them collectable as well as just a bullion coin. You will struggle to find every year back to the 80's. so again the prices go up.