I start mine off in a 500ml yoghurt pot, so it does not take up much room. I suppose you could do this and put a plastic bag over the top and keep at at your allotment.
I never seem to get them to pencil size, but it does not seem to matter, once they are planted out the soon get going.
I simply take out the plants and wash off the potting compost, trim off some of the root, and throw them down a hole.
I have sown some at the allotment and then moved them. This worked ok. They are also quite happy germinating under a plastic bottle, so keep off pests.
I find they need to go deeper into the soil to get a longer white section, so do need transplanting.