There's a great cut and come again Chinese brassica called Mizuna Greens (also Mibuna greens which are similar but not as good). If you sow these in modules and plant out as soon as you can, they make a great supply of green leaves you can cut when there's almost no other brassicas. Flavour's stronger than cabbage but still very good. If you cut them the growth will come back after a couple of weeks and you can keep doing this for several cuts well into the spring.
The Suttons cabbage 'Advantage' is also very good and can be got to an eatable size early on from sowings in modules in early January.
My favourite would also be the little white turnips like the Japanese ones, or the French variety Long de Croissy which has long roots like a radish. These are beautifully sweet and grow really fast in spring.
Another one we do, is what the Italians called Cima di Rapa (which is actually a turnip green type which doesn't form a turnip!). Seeds of Italy used to do several varieties which matured after 40, 60 or 90 days. The 90 day version was best as the little flowering shoots which look like broccoli, were best on the longer maturing one. However, don't expect this to taste like broccoli as it has a very strong brassica flavour.
These are some of the quickest to mature, but there are lots of other things you could be planting out at this time which would mature much later.

What you plant in February or March, Guest, firstly depend on just where you live in the UK. In the mild west you would be planting much earlier than in the east. Then, taking your question literaly, there is possibly not a lot to 'plant out' at that time of year. Sowing seeds would be a different matter.
Where I live I would not expect to plant or sow anything outside in February. In March, weather permitting, I would hope to sow broad beans, early peas and parsnips along with onion sets and early potatoes (fingers crossed) towards the end of the month. I consider these to be 'easy' in my garden but may not be so suitable in yours.