“Why do people think they need to invest the time (and expense!) into feeding them their foul cakes?”
What a weird response. What is your issue? You don’t have to eat the baking. Do you have issues with food or do you perceive this woman as a threat? Maybe she is practising on her workmates because she has a place in the next Bake Off. One of my friends used to work with a Bake Off contestant and she said he was always bringing his creations into work.
I occasionally bring baking into work. In fact people now ask me to. One of my team is leaving soon and he has hinted that she would like me to make a lemon drizzle cake.
“Am I the only one that thinks this is weird then?? I admit I don’t like cake at all”
Yep. You’re odd.
“We have people at work who are constantly baking too. I just accept their offerings with grace and then bin them on the way home. I don't eat sugary stuff.”
Why not just refuse the baking then? 
Just tell the baker you don’t eat sugary stuff and let someone else have another cake. That is such a bizarre and wasteful thing to do.
“but I STILL don’t get the constant baking!! Its obsessive expensive and compulsive! I am clearly massively out of touch”
It doesn’t have to be expensive, and yes, you are massively out of touch. Are you not even aware of who Marry Berry is? Have you even heard of the GBBO? Baking as a hobby has massively taken off over the last few years.
“I would still find the whole dogged ritual bloody weird.”
It isn’t. It really isn’t. How long is it since you worked in an office with other people?
I suspect your husband enjoys her baking because he will never get home baked goods at home.