Well, you can start onions now, the problem is going to be dampness during Jan and Feb, they are likely to get damping off if you do.
You could try spinach and cut and come again salads but the same applies.
If you have space with light and heat, you can start chilli's off now, but you won't be able to put them outside really until May or June, so you will have to have space for them indoors until then.
The issue in the UK is damping off and lack of sunlight as much as anything.
I usually start chillis off in Jan, in a heated propagator, then transplant and keep them in an outdoor greenhouse on a heat mat. It's just this year with moving, no greenhouse yet so I'll have to get something sorted in the new house for them for next month.
Then in Feb I'll do a few bush tomatoes with the same heating arrangements. And later Feb I'll start sowing the more hardier stuff, like spring onions, main onions, spinach, lettuces, beetroots, chard, fennel, celtuce, peas, mangetout, kohl rabi.
In march the rest of the tomatoes (I grew 50 varieties, 2 plants of each last year), and another sowing of much of the Feb list. Plus loads of carrots, parsnips and leeks.
In April starts the more tender stuff, so a few early courgettes with the main tender stuff, courgettes, squashes and beans at the start of May. Also a round of annual herbs, basil and corianders and annual flowers such as tagetes and nasturtiums.
Then the main planting out into the polytunnel gets underway, and when that is done, another round of the Feb lot, plus sweetcorn and some of my other weird stuff like achocha, yard long beans etc, another round of peas, fill in gaps with spare beans.
June and July, not alot sowing, but August my winter sowing starts, with kales, spring onions, kohl rabis, fennel, coriander, Pak Choi, and any of the Asian leaves, a winter lettuce or 3.
Come mid September and garlics can start to be put into some of the spaces being vacated by harvests, and start chopping all the dying leaves for the compost heap. A good mulch for the winter and sow spinach in every spare space/bed, at least 3 different varieties, and start transplanting out all the August sown crops. Also don't forget claytonia, miners lettuce, one of the best crops for winter as it is indestructible and self seeds everywhere. Excellent little crops that. Sow September if you can get seeds.
October the last of the toms and chillis usually around now, so lots of seed saving and drying going on, carry on planting August grown crops and making compost. You can also at this point put overwintering onions in, which should crop April/May (small) or June (larger).
November just keep on top of any additional dying stuff, turning the compost, and mulching. Pak choi is cropped and eaten, same as most of the fennel. And some kohl rabi. Carrots and parsnips are all out now, parsnips after the first frost.
December, plenty of kales, salads, corianders and spinaches should be being produced. I have been able to pick enough for greens in every meal before we left the UK, pick a huge batch once a week and wash, and keep in the bottom of the fridge for chucking into anything.
Then the next Jan and Feb your winter crops still keep coming, and at the start of march and into April, your spring onions will start to be thick enough to pick. Kohl rabis and those leeks also thicken up. Pick and eat often at this point or they will start to flower.
Then your beds should start to be emptied from those winter crops in time for the next ones that you sowed in Feb and March to be transplanted into, or sowed direct.
That's pretty much my cycle. I have just moved countries so am starting from scratch, but in the last 6 weeks have sown chard, spinach and kale, and have garlic in, and I'm about to start the January sowing regime once I have the heat mat set up. I also have to put my polytunnel back up, but we have to wait until the trees start to leaf up again as we are trying to be discreet and we are in the line of sight of the Mairie. So we won't be doing that until at least April.