For sweet potatoes there have been some useful threads with growing tips - quite different from normal potatoes, use your search button and all will be revealed!
For normal potatoes, you can plant them outside in April / May or so and protect them from frost by earthing up (covering with soil) as they grow, and by the time you let the foliage grow properly above ground the risk of frost should be pretty much over. In most parts of the country anyway!
For Christmas potatoes (which are a bit of a hit-and-hope thing anyway, by all accounts) there is more of a risk of frost killing them before the spuds are ready, so might not work to plant them outside now. I've got Christmas potatoes planted in pots to bring into the greenhouse, although saying that I haven't seen any signs of life yet!