Les, can I please comment on some of your points?
The BLM "confiscation" fear you cite, would only apply to things found on federal land (not state, county, city, or private). And would only apply to certain items. Eg.: archaeologically significant, which is generally deemed to be 50 (or 100) yrs. old or older. And even THEN some would argue semantics that bullets and coins are "exempt". (although that latter point, is up for debate, as the key is whether or not they were in context with something cultural, etc.... or not). And really Les, even all THIS assumes that some fed. archie is there, with calculator in-hand, studying all the items in your apron. I mean ... seriously ... when was the last time you were in the middle of the forest/BLM or desert or something, with "concerned officials" out there assessing your each find?

Sure, I don't doubt that such "fears" are something to be concerned about, if any of us are snooping around obvious historic monuments, or if you're in front of some "purist" archie who just happened to be waltzing by.
I mean, take for example your "arrowhead" example. While it sounds so horrible (and now we're all supposed to be leery of archies who "lurk around every corner and hide under every rock waiting to pounce on us). Yet that, to me, is akin to if I found a news story of some motorist who got pulled over, and roughed up, by an overzealous cop, for nothing but his tail-light out (and threatened with seizing his car, thrown in jail, ticketed, etc.... all for a "tail-light out"). Could it happen? SURE! There's ALWAYS some extreme example of someone getting "roughed up". But there comes a point when you can treat some stuff as isolated. If all such extreme stories are to be the standard by which we govern our future md'ing places, then you might as well give it up now. Because I bet I can find a story of someone roughed up and thrown in jail for hunting a sandbox at a city park and finding a clad dime.
Sure, I'm not saying "go flaunt your activities in front of purist archies". But sheesk, can't you be a little discreet and avoid those 1 in a million people?
As for the laws in Britain, I'm not sure what your point is. Are you trying to paint a rosy picture about "wouldn't it be wonderful to have such a system here?". If so, you're over-looking several key differences of their laws, verses our laws, that predate and have nothing to do with detecting (and would render it impossible to ever happen here). Namely, that in the UK, everything under the ground (even on private land) belongs to the queen. So for example: in the USA, if you discover oil on your land, your rich, right? (like the beverly hillbillies show, haha). But in the UK, if a farmer discovers oil on his land, the UK govt. gets it all, and you are dealt with on their terms, not the open market. Thus is the same for those metal objects under the ground. The only time the farmer or md'r can keep them, is if the govt. isn't interested in them (and "lets you" have them). So they don't declare and give individual coins, for example, unless something stupendous. Thus usually hordes are given/sold to the govt.). However, in the USA, if you find a cache on your land (or an md'r finds a cache on farmers land), it's theirs to keep and do what you want with. Thus it seems to me that the USA system is the preferred system, not the UK system.
And in the UK, their "protected" sites are not un-like the USA's "protected sites" (federal land, etc...). Thus those UK hunters you read about that get compensated (and a "thankyou") from the governenment are NOT finding those things on UK federal land. They're finding them on private farmers land (someone correct me if I'm wrong). There are HOARDS of sites in the UK on federal/public land that you CAN'T detect on, just like in the USA. So don't wax romantic about the UK system too quickly.