I don't have experience in this type of Visa, but your post prompted some random thoughts.
Visa aside, I was wondering if it might be possible to find a job stateside that placed you in the UK. (ie: working in the travel section of a newspaper; moving into magazine photography for a freelance assignment, etc.)
I mention this aspect because I when first came to the UK, not to live, but to visit, I was working as a freelance artist (web design)...I was working for a US company, but because of the magic of the Internet and FedEx, they really didn't care where I was, as long as I got the work done. So, even though I was in the UK, I was earning US funds, which were deposited straight into my US account and I pulled out at ATM machines. And I didn't stay for long.
I mention these temporary possibilities because you're right, it's very hard to get sponsored for a Work Visa in the way you want to go about doing it.
If you are wanting to move permanently to England and have no intention of marrying your boyfriend (if he's British), then the temporary option isn't going to be for you. I was merely suggesting this as a way to see if you liked England enough to live here, see if you liked your bf enough to marry him...but it is only temporary and you would not be able to earn any UK funds, nor have many other benefits.