Welcome to Flyertalk and welcome to the BA forum MWF. By the sounds of it, you may not find yourself visiting here too often, but there are advantages if you persist. Not least finding the ways and means to get best value from your travel spend.

There is no real point in buy seating on CE, the nature of Theoretical Seating is such is that she will almost certainly have a selection of seats available at online check-in, including perhaps row 1. I can see some point in buying seat selection in ET if in a group, but not in CE as a single traveller. The original ticket is something of a bargain, and if your partner gets lounge access, extra baggage, Premium Gatwick security, extra tier points and Avios, then the uplift for Club Europe seems reasonable or even good value for money. So stop the money machine there, would be my take on it, seat selection pricing makes your ticket cheaper, so long as other people pay it!