Welcome to FlyerTalk, Matthew!
This does get confusing. To review the fare issues:
W,V,Q are the cheapest discount ticket "fare bases." These can never be upgraded with miles.
H,M are the next lot up. They can be upgraded for 20,000 miles each way from Europe to the US.
B,Y are unrestricted or "full-fare" economy. They can be upgraded for 10,000 miles each way. (B is actually so minimally restricted as to be comparable to pure Y.)
Personally, I've never found an upgradeable ticket at a reasonable price; I would
never pay as much as H fare. As a rule of thumb, "H" is at least twice the lowest discount fare, usually ~ $1,000 SFO-LHR. But some people find it a fair price to pay for the fewer restrictions, include the ability to upgrade.
American Airlines, by contrast, allows flyers to upgrade using miles on
any published fare basis.
United always gives full mileage credit as long as you pay
any published fare. The credit you receive depends on the fare basis of your underlying ticket, apart from upgrades: 100% for economy, 125% for paid business.
As pointed out above, the main differences between upgrading and flying XC is that the first method allows you to accrue miles while the other entitles you to use Arrivals by United (shower, suit press, breakfast).
Availability of upgrade seats can be checked using United Connection 3.0, I believe.
http://members.aol.com/premexfiles/ucweb30.exe Also, the software for the Palm Pilot does this.
For calculation purposes, base mileage from Heathrow to SFO is 10,736 return; 11,736 for Reno.