I wouldn't plant anything this year. I'd prepare for next.
A lot depends on your soil, and what you want to grow as to how you prepare it. If it's veg you are growing, your allotment should be split into 3 or 4 sections to allow for crop rotation. Just google that and follow what it says.
And whatever the roots are, keep digging them out.

You have got to get the roots of the weeds/shrubs out or they will re-grow. Too late to start growing veggies this year so work on improving the soil.

Get some tests done on it to see what it needs, and what the base mix is that you are starting with--sand, silt, clay, loam, etc. And, as everyone says, sift those roots out, for you will really regret it if you don't, especially if they are perennial weeds, such as nettles, brambles, and the like. Every hour of work that you put in on the soil now will save you ten hours of work (or more!) in the years to come.