Vermiculite for raised beds
I'm going to give gardening in Brighton a go this year & as you know there's no soil here, it's just clay. So I think I will try raised beds with a mix of equal parts compost, sphagnum moss & vermiculite. Do you think this will work? And where can I buy bulk vermiculite & moss? What is an ideal depth for veggies, flowers?
Amending a clay soil to make it more favorable for plant growth is generally accomplished through additions of organic matter. The peat moss you are considering or another plant based compost would be good choices. Over time as such organic amendments are broken down in the soil they release nutrients and help aggregated ("break-up") clay particles. This means the soil will be better aerated, have improved drainage, allow greater root spread and generally be better for plant growth. Add 2-3 inches of compost to the top of your soil and then mix it 6-8 inches deep. This should be done yearly in veggie beds.
The vermiculite is not something I would add to clay soil. Vermiculite holds a lot of water which is why it is used an an amendment in sandy soils which do not hold a lot of water. Clay soils can hold a lot of water already. So it may not hurt but it is unlikely to help to in a clay soil and can be quite expensive.
Here are some fact sheets if you would like more detailed information:
Hope this helps please let me know if you have any further questions,