you could:
1) group your onion varieties together (that is, spring onion, seed onion) Leeks could go in this group, but if you are planting 1st early potatoes, you could wait until you've dug them up late June/early July and by then the leek seedlings will be big enough to handle and could be planted out in that bed instead.
2) the turnips and sprouts together (but do note that brussel sprouts will need about 2 feet space in each direction to grow, and grow about 2-3 feet tall so make sure the others don't find themselves in shade and overcrowded - see if you can locate south and sow the turnip seeds on that side of the bed) Please think about erecting some sort of netted structure to protect your brussel sprouts from pests such as cabbage butterflies (the caterpillars can do a devastating job on brassicas) and pigeons. Sprouts need a long growing season so worth protecting early on.
3) Fennel could go with the salad stuff.
4) Butternut squash like all trailing squash are planted out about 3 feet apart, and needs space to roam so a big bed would be preferable and also grow more than one plant to ensure flowers have a better chance of pollination.
Not sure what you meant by "channelling" potatoes - presumably how far apart are the rows? It depends on whether you have first earlies, second earlies or maincrop. Maincrop about 2 feet between rows, 18-24" between plants, first earlies you can squeeze them in a little tighter say 18" between rows with about 12-18" between plants, with 2nd earlies somewhere in between the 2.
