Hi Janie - I'm not very lucky with seeds - well not very good with them! I like to buy small plants from naturescape and grow them on. Some of them work so well that they seed themselves around the garden.
http://www.naturescape.co.uk/acatalog/
A cold frame is great. I am also in a bad weather area (the west of scotland) and a coldframe makes a huge difference. When the sun hits the coldframe the temperature shoots up. Be sure to always have some shade inside the coldframe as it can get dangerously hot.