A couple of things at play.
- yes many of the WTP fares are actually an upgrade from economy, so the taxes and fuel charges can jump a little
- taxes and other charges are higher in club than WTP. The tax element is a U.K. tax on premium bookings so one we cannot blame BA for.
- you could do an ex EU to bring the price down. Both due to lower taxes as a transit pax and BA competing with other airlines.
in terms of the upgrade you have interpreted corrected. You can have you travel agent or BA do it all at one time. In that way you will be confirmed at time of booking.
KF