I would think if one is looking at coins as a long term investment then that person would need a combination of many things; knowledge of the subject, the money to make considered purchases and then be able to have that money tied up in something long enough to see the returns, and a large pinch of luck being in the right place at the right time to buy the right coins. If that person has all of that and possibly other traits I am not aware of then they should go for it.
Personally I do not see coin collecting as a money making investment for the future, a £70 coin will probably not be worth huge amounts even after 20 years. However I do believe there is the potential to make short term profits by looking at the trends and buying at the right price along with a bit of luck.
Just my opinion, and please do not take it in any other way. Keep enjoying the hobby