It is my opinion that part of having a successful long-term relationship is being committed to the other person's success and well-being.
This commitment is a form of investment in and of itself. The returns are typically non-monetary, so it's important to understand what money actually is.
Money is a token people exchange for favors.
If I go to a deli and ask for a sandwich. I give them tokens for the favor of having received a sandwich. The people at the deli then exchange those tokens for other favors, and that's the entire economy: people doing favors for other people in exchange for tokens that represent more favors.
Sometimes being invested in your spouse is giving them a back rub when they've had a hard day. The investment pays off when you have a hard day and they give you a back rub.
Sometimes being invested in your spouse is taking them to a masseuse for a professional massage. The investment pays off when they get two tickets to that thing you love.
At the small scale it's easy to mostly ignore minor monetary discrepancies.
At the large scale (which I think £50k is plenty large enough given your listed net worth) it becomes harder to tell if the opportunity cost will be worth making that investment. It pretty much comes down to:
Will the quality-of-life improvements from that investment be better than the quality-of-life improvements you receive from investing that money elsewhere?
As far as answering your actual question of:
How should I proceed?
There isn't a one-size fits all answer to this. It comes down to decisions you have to make, such as:
- do you think this opportunity is worth the risk?
- do you want to treat your spouse as an economic adversary or economic partner?*
- are there other alternatives that you can explore to make the situation more equitable?
* in theory it's easy to say that everyone should be able to trust their spouse, but in practice there are a lot of people who are very bad at handling money. It can be worthwhile in some instances to keep your spouse at an arms length from your finances for their own good, such as if your spouse has a gambling addiction.
With all of that said, it sounds like you're living in a £1.5m house rent-free. How much of an opportunity cost is that to your wife? Has she been freely investing in your well-being with no explicit expectation of being repaid? This can be your chance to provide a return on her investment.
If it were me, I'd make the investment in my spouse, and consider it "rent" while enjoying the improvements to my quality of life that come with it.