because of the ridiculous british weather i'd imagine most land owners haven't got the amount of land needed for 24/7 grazing when horses will churn up the land in wet weather.
also the amount of maintenance that needs doing on these fields probably isn't worth it financially with the amount they get paid for grass livery
I am going to be offering full time turnout all year but with a stable "just in case" for £20 per week - the problems with just doing grass livery is if you have stables you can only have so many on the land but people want to pay less for grass livery. As a potential Yard Manager it would worry me that people would have no where to stable in an emergency and nowhere for farrier etc.
If you are literally looking for a field it may be worth contacting local farmers.