Getting a conviction for it would be hard.
Say in my case, I worked full time. Earned decent money. Exh told me that my share of the bills was 'X amount' which keft me with basically enough money for petrol and about £30 a week, for things like work clothes, it I wanted to go out, wanted to get my daughter something etc. I had the basics like food, clothes (But not many especially for work), a car etc.
I didn't question my half of the bills because I had been condition to not question him.
It turned out that not only was I paying all the bills, he was earning far more than me, far more than he said and had money left from what I have him to put in savings. So he was saving his whole wage plus a bit of what I have him. I was paying everything. Plus a bit on top.
When I got a wage rise he said 'great, that extra can go towards bills so I took on more responsibility at work and didn't see any of it.
He tried counselling as our marriage was splitting up and admitted these things. That when it came out about how much he was earning at the time. When I asked why he insisted on taking so much from me the only answer he could give was 'I don't know'.
Anyway when I wouldn't accept these answers it spilled into physical abuse, so perhaps the financial abuse issue was less important (legally and in some people's opinions).
But would that be not classed as financial abuse. You could argue that I chose to give him my wage, that I chose to not find out how much he earned, how much the bills were etc. But financial abuse doesn't exist in a vacuum, there's lots of cohesion and emotional abuse with it that makes those choices not feel like choices at all.