I am afraid there is no way to change the capital gain position - it is unfair I cannot disagree with you , but sadly as soon as 18 months lapse from the time this is deemed to be your residence, then capital gains comes into play.
This could have been avoided had an agreement regarding the property be drawn up in the tax year of separation, as then the legislation set would have been treated as arising in the same year - regardless of when the sale/transfer actually took place.
But on the plus side here value of the property when you do transfer/sell less the value at purcahse, will be small as its a recent purcahse (just over 2 years apprx)
So you are NOT liable to the £75,000 you hope to get - but 50% of the net value
So say you bought for £200,000 and when the transfer/sale takes place you get £300,000 then you considered as half of the profit which is only £50,000 (I appreciate you will just get 25% but as shared owner you are treated as 50% owner)
If there was no mortgage in place you could make an election now to reflect that the share is other than 50:50 - but you did indicate there was still a mortgage in place - but if its possible to change this the tax year PRIOR to that of the sale, then your capital gain position would be reflected on your actual legal share.
Let me know if I can assist further, but if you have all that you need, then it would be appreciated if you could rate me for the level of service I have provided (or click accept)