If you then take maintenance into account, the MCR is likely to cost you around £5k per year less than your 172.
It's not my 172, but if it were, I'd be flying it for several years while you're building your MCR, assuming you ever finish your airplane, which many homebuilders never do. I'll also own it for substantially less; I can afford a lot of years at five grand a year, to make up the difference, and I'll have a complete, flying airplane, whereas you'll receive a box of raw materials and parts.
Let's see how well the MCR holds up to the rigors of several thousand hours of student training. And charter, and...no, wait a tick. You can't charter it, and student training will be limited to non-existent. Not enough of them out there, and it's not a certified design built to known production standards by a reputable manufacturer.
You can go build your MCR, and have a one-of-a-kind airplane, or you can buy a much less expensive airplane, be ready to go, with something that can actually earn it's keep. It's up to you, of course.