You can use the cold frame to harden them off starting a few weeks before when you expect the last frost to be (late May here, possibly earlier for you if you're further south or on the coast), closing it down at night to begin with and covering with fleece or newspaper or cardboard or old curtains etc if there's frost forecast.
Like the others have said, it's best to avoid sowing too early if like me you don't have a greenhouse, or have one but don't heat it (expensive these days). I've just sown a few chillies because they do need a long season but I won't be doing any more of the tender stuff until April, and the things that get big very fast (cucumbers, courgettes, ipomea) will be sown even later. I've only got one suitable windowsill indoors and an old table that I stand in front of it with a foil-lined reflector box and some growlights, so I have to be choosy about what I can start early.
This sounds like my set up too. Never thought I'd be longing for my 1980s conservatory again!
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