In the United States, Federal Government entities such as the US Forest Service have intentionally misinterpreted the Archaeological Resoure and Protection Act to basically make criminals out of metal detector users. Most Federal Lands (Public Lands) are off limits to detecting, or at the very least have restrictions placed on detecting. Most State Parks ban detecting as well.
Over here, there is actually a movement that wants to "preserve" pre-historic and significant historical sites that are privately owned.Though this sounds benign on the outside, the real issue is that an archaeologist could designate a person's private property as a pre-historic site and once that designation is given, the land owner would be forbidden to build, excavate, or even sell that property.
Furthermore, we don't have the laws which you have that actually encourage landowners to grant permission to detectorists, and the system that allows the detectorist and landowner to share in the value of the finds. Though there are isolated cases of Archaeologists grudgingly using volunteer detectorists on certain historical sites, many Archaeologists loathe us, and label us looters, pothunters, and thieves.
Sometimes the pen is mightier than the sword. What about starting a writing campaign across England, pointing out the many significant historical finds that ordinary people have made, and how greatly the Archaeology community--and England herself--have benefitted from the history that people like you have uncovered and brought to light. Point out the benefits that metal detecting clubs and organizations make in preserving England's history. Maybe some of your clubs should use the media to suggest that Britons detectorists actually save more history than the archaeologists do.
And here's something else to consider: Isn't it possible that in the last 10 years or so, detectorists have saved more of England's history than its archaeologists have--and have filled your museums with more artifacts as well? Furthermore, I'd venture that there's more information on the internet available about England's history from metal detector sources than archaeological ones.
When archaeologists uncover artifacts, they write a periodical about the dig, which gets buried deep in a Univeristy Library, a few of the artifacts go on display--briefly--then pretty much all of the artifacts are boxed up and placed in the basement of this university or that museum, where only a select few people are privy to them. Many, many detectorists display their finds for public scrutiny--and post them on the internet as well, and I'd be willing to bet that detectorists are responsible for more artifact specific publications than the archaeologists are.
Don't be depressed. If you are faced with some of that misinformation when you ask permission, kindly and gently tell the landowner what the TRUTH is. Take the opportunity to REALLY educate the public. I think that if all of the detectorists in Engand banded together and publicized this issue, perhaps you might find that you have more allies than you realize.
hope you find a hoard of gold 
Kurt