Is it possible to buy someone out of a house with a 2nd mortgage / remortgage?

Unfortunately my partner and I are separating. We brought a house for 400k and have a 200k mortgage currently on a fix ending Dec 2026 and we own the house 50/50. We are exploring options around the house although haven't yet spoken to an advisor.

Is it possible for me to remain in the house and buy them out by essentially taking out a second mortgage for their 100k and releasing their equity (assuming this means no ERC)? Alternatively, is it possible to remortgage from 200k to 300k and change deeds into my name only (I would meet affordability of a 300k mortgage). I assume this remortgage would require payment of ERC as its during the fix period?

Any thoughts / experiences around this would be helpful!