Kohlrabi

I don't think they would last well on a windowsill until October. They don't mind being transplanted but it's probably best to do when they're relatively young. Like mcdood says, they won't do much after October. They get a foot or more tall so you'd need a pot of decent volume.

For planting in October you're likely to do better with things like spinach, pak choi, radish, winter radish, mustard greens, baby leaf kale, winter lettuce, spring onions and rocket. Sown in August/early September they can go into a greenhouse or under cover outdoors in October and crop well over winter. Lambs lettuce and miner's lettuce can stay outside without cover. Frost hardy cabbages like Winterjewel and Spring Hero can stay outside all winter for cropping in April/May. Cabbages, cauliflower and perhaps calabrese can all overwinter in pots in a greenhouse to be planted out in early spring.