Onions require a lot of light to do well and not be 'floppy'. They really need to be outside or under strong indoor lights, not just a windowsill except that they can sprout there just fine.

I don't know anything about the weather in the UK so I don't know if it is the right time for onions outside or not.

Emma,
My Daughter is in Derbey and she says its been very wet. So humitity is high and if your window area is not well heated and dry, you should be careful not to water unless needed. Once growing well, they will do best in natural light. You can prune the tops at 10 centemeters which will encourage them to be less "floppy. Onion seeds will also sprout faster if they have bottom heat.
Now a question for you. About 20 years ago, my wifes Mother visited England. She says that many people had tomatoes growing on their window sills. And that the tomatoes were quite good. Can you tell me what varity those would be?
Thank you.

Hi,

Thanks for your help. I grow my tomatoes outdoors, so I don't think I can help, sorry. Have you tried looking at Thomson and Morgan the seed company? As you may see the tomatoes there.

Take Care
Emma

I think you need to re-start your seed in spring. Probably February, and set out in late March early April.

Unless this variety is adapted to over-wintering, in which case you should set them out in jig time. You might want to do that in any case, because you are going to get nothing with them indoors. Outdoors and over-winter them and at least you will get more seed next summer.

Emma, I have done some searchs includeing Thompson and Morgan in the UK. All seem to point to setting your plants out in April and starting the seeds in mid winter. If you purchased your onion seeds in the UK, what did the instructions say? It would certainly depend on your local climate.

This link seems to have the most information on onions in the UK.
Another option that I had forgotten are the heat treated onion sets which are not susposed to bolt to seed. Would like to try them here but don't seem to be available.

Here is a link that might be useful: How to grow onions in the UK