Welcome to Flyertalk,
matthewuk!
To answer your first question...it really depends on your situation.
If you've gotten totally sick of flying UA and plan on doing business with BA/VA in the future, and would rather spend those earned miles, I would say, yes, spend that 80K for a free Biz class transatlantic ticket.
But if you plan on bringing more of your flying business to UA, then, hands down, get yourself an upgradable Economy ticket (H fare and higher), and upgrade with 20K/way. I personally consider upgrading an Int'l flight (esp. those long-a** transpacific ones) a great value for your miles. Remember, you may be spending 40K on roundtrip upgrade, but LHR-JFK (or whereever your destination is in the US) is at least 5000 miles one way, so you're only giving up about 30K really. A lot less if you have other bonus miles (like your Premier bonus, etc)
Another thing to consider...flights from UK to the U.S. are often cheaper than the other way around, so you already have an upper hand over us Yanks. This is true especially during the low season (winter). Just book your flights and reserve your upgrade space 1-2 months in advance. (you said you normally book 4 months in advance..keep doing what you're doing)
Yes, you still get miles for upgraded travel, but only on the class paid. (i.e. if you buy Economy tix and upgrade to Biz, you get miles for Economy, which is the actual miles flown plus that 25% Premier bonus, if you fly on UA metal, LH too, I believe)
From what I understand, Biz Saver Award and Biz Upgrade comes from the same bucket, which is called the
NC bucket. Can't really say what your chances are like; it all depends on whether you're flying weekdays/weekends, low/high season, etc. You may call up Int'l Mileage Plus Reservation and check the NC inventory for the route and dates you want.
My success so far is 100% on Int'l flights when I book H fare; but often enough I have to be flexible with my travel dates (plus minus 1-2 days) just that sometimes I have to cringe for the delta between the most el-cheapo Int'l fare with the upgradable Int'l fare; which, for U.S. - Europe, can be 100% difference. (i.e. twice as high)
Is it worth it? If you're tall and need space to sleep, eat, work, or to even sit comfortably, YES!!
Some people here complain that UA Biz Transatlantic are just so-so, but I'm an easy pax, so I rate it two thumbs up for the money and miles spent. Well, except when it's summertime when H fare to Europe is usually above US$1000 so price isn't a real winner. If you want GOOD GOOD service with free massage on board, etc etc, you fly Virgin. No idea how much it cost to get upgraded though, but probably more than my annual income.
Hope this helps, sorry for the rambling!
Jamester