Using "Book and Upgrade" to upgrade British Airways flights

Last summer I did a mileage redemption to fly one-way JFK to Heathrow in British Airways' Business (Club World). It cost me 40,000 points + $437 in fees. While I'm no fan of BA's high fuel surcharges, it was still a great deal for the upper deck of a 747. 

Overnight Club World sleeper service in the upper deck of a BA 747

I've been reading a lot lately about using BA's "Book and Upgrade" feature as a better use of your money/miles than a straight-up mileage reward seat. BA lets you use miles to upgrade one class of service, so you're essentially buying a ticket in Premium Economy with cash and then using miles to upgrade it to Business. This is a clunky process on many other airlines' sites, but BA has a nice interface to help you find an upgradeable ticket to purchase and then instantly upgrade in one step. I'm not going to price out the Economy-to-Economy Plus option because it's not something I'd bother with upgrading to, nor the Business-to-First option because I'd never pay full price for a business class ticket in the first place. 

Click the Book and Upgrade tab to search for instant upgrades

Summary

I'll put the details below but here's my takeaway:

1. Book and Upgrade is a bad deal for 1-ways, use a pure-mileage redemption instead.
2. I'm sure there are differences in availability between pure mileage vs. B&U, so check out both options if you're trying to fly on specific days and have enough miles to be flexible. 
3. If you don't have many miles in your account Book and Upgrade is a good option
4. Remember that BA caps the number of miles you can buy per calendar year at 24,000

Details

When you book a trip purely with miles, BA has a unique feature that shows you several options on a cash/points continuum, so to me it feels like Book and Upgrade is just another point on that same scale. 

In this case, a roundtrip NYC–London pure miles ticket is 80,000 points + $1204 in fees. If you look at the last of the options in the picture, you can "keep" 40,000 of your points if you're willing to pay an extra $1120 in cash. If you think about it, they're essentially "selling" you 40,000 points at 2.8¢ each that don't count against your 24,000 per year cap. (BA seems to charge around 2.8¢ whenever they sell you miles). 

80,000 points + $1204 or choose one of these other options... 

Now compare that to Book and Upgrade: 20,000 miles + $2277 — a great deal compared to 40,000 + $2324 above. So assuming you can find an open seat on a trip that fits your needs, the Book and Upgrade option is "selling" you the 60,000 mile difference for 1.8¢ a piece – a full cent cheaper per mile than any of the mileage-only discount options.

(FYI, I tried pricing out the one-way options here and, unfortunately, it was a much worse deal: 10,000 points but $2130 in fees – half the miles but almost the same amount of cash! I guess this makes sense since one-way fares across the Atlantic are often as expensive as roundtrips and you're technically buying a cash fare here.)