I like Real Seeds, Dobies (not the same as Dobbies, but both into gardening), and Victoriana Nursery for online seeds.
I take it you will be growing in a greenhouse or polytunnel?  If not, you'll definitely want cloches, or a really sunny/warm space.  
Chillies need a long start, so late January/early February is good. Definitely need heat - so indoors, heated propagator, or heat mats are your best bet. 
Tomatoes you can start a bit later (March/April).  Lettuce/spinach you can sow whenever is starts to get slightly warm, then pretty much all spring and late summer/autumn, little and often is good.