Sowing anything into the open ground now - even under cloches - is pretty much a waste of time in my experience, as the weather is so unpredictable over recent winters that even if it germinates and grows on well for a while - poor weather later on in winter may destroy it and waste expensive seed. That is unless you live in a very mild area, with very well drained soil and don't have a slug problem (is there anyone who doesn't?. If you're lucky enough to have a well drained, warm soil and are desperate to sow something - you could sow varieties of overwintering broad beans and peas outside - but I've always found that sowing into modules or pots of peat-free co,post in a greenhouse or on your windowsill in late Jan. or early Feb. next year will produce far healthier plants, with an equally early and usually far heavier crop. It's always far safer on my heavy soil - avoiding the risk of possible plant losses through unpredictable weather or slugs. Few of us want to spend cold evenings outdoors slug hunting - and they're still active unless the winter is bitterly cold! Spending a bit of time in the warm planning next year's rotations really well and choosing what varieties you want to grow is a far more useful and productive way to spend one's time!
If you have a sheltered cold frame, greenhouse or tunnel
You could still sow suitable types of winter lettuces like 'Winter Gem Vaila' (little gem type), 'Rosetta' - (a reliable indoor winter butter head), and also old fashioned varieties such as 'Black Seeded Simpson' a huge loose-leaved butter head type which can be picked a leaf at a time. Seed is expensive and with expensive F1 seeds you don't get many in a packet - so sowing individually into modules is by far the most cost-effective method even if your tunnel soil is still warm enough for germination of direct sowings in the soil. Sowing into modules also reduces the risk of slug or early woodlouse damage and provides better air circulation - thereby preventing 'damping off' diseases. Even the cheap 'value' lettuce or other salad mixes can still be successful sown thinly in early Nov. - I've often harvested these in the polytunnel until the following May! The reason those mixes are cheaper is because they are usually older, tried and tested, 'bog-standard' open-pollinated varieties (cheaper to produce), which can often be more disease and cold-resistant than expensively bred 'F1' hybrids.
Garlic cloves can be sown/planted now - both outside and also in tunnels for a really early crop of big bulbs next year - most varieties need cold weather for good root development. Choose firm, plump and healthy outside cloves from this year's crop, or buy certified virus-free ones from garden centres - not supermarket bought bulbs which will be unsuitable for this climate and may even bring in diseases like onion white rot - this can survive in the soil for up to 20 years and be spread around the garden on your boots - infecting all members of the onion family including leeks! For the same reason I don't use onion sets in the vegetable garden. If I want some extra early onions - then I grow some sets in pots or containers. This way they're much earlier than any grown in the ground - and if you're unlucky enough to get any disease you can just throw the remains, along with the compost they were grown in, into the food/green waste recycling bin - rather than spreading it around the garden! I grow all my main crop onions from seed sown in early March - it's very easy and by doing this I avoid the possibility of onion white rot. Seed sown onions also are far less likely to 'bolt' in difficult weather - a major problem this year - and they always keep far better. Mine always keep until well into spring - if they last that long!