Depends from council to council what they do with the Green Waste. London Borough of Hillingdon have a very efficient operation ongoing with an outsourced contractor, but you can only get a small amount of free stuff each year – nothing like enough to cover an allotment.
I go around filling my green waste bags with fallen leaves at around this time of year and that is a better mulch than nothing if you can’t get any manure this year. I”m currently experimenting using different mulches as preparation for different crops – I use horse manure for potatoes, this year I’m trying leaf mulch where the parsnips, carrots and onions will go and I’ve used home made compost for the garlic, spring cabbage, winter onions and spinach.