If you are buying bullion for the sake of bullion, the Royal Mint is the last place to start. Even with their discounts their prices are way over bullion value, so it will be years before you even break even. You would do better through any of the numerous bullion sellers online where you pay close to the current gold or silver value for the coin.
Even if you are getting bullion but also like the coins, you would do better through dealers and coin fairs, where you can get very decent sovereigns etc from previous years at not much over bullion value. (I was shown 20 Sovereigns a few days ago in a range of dates back to Victoria and all VF or better, blanket price £250 each.)
Personally I would forget gambling on the commodity market (which is all that bullion buying really is) and spend some time at a coin fair buying coins you really like and have a bit of history!