Let me take a look at this and I'll get back to you in a bit with my answer.
Assuming that the buy out will occur after the end of the tax year in which you separated, you will be treated as having bought your husband out at the market value of his share of the property regardless what you actually pay him and if that is higher than his share of the original purchase price, then he may have CGT to pay. The first £11,100 of gains an individual makes in a tax year are tax free.
Take a look at the notes here and here on separation and divorce. There can be a variation in the deemed disposal proceeds depending when in the court/divorce process the sale is treated as having taken place.
I hope this helps but let me know if you have any further questions.