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HMRC link for reference https://www.gov.uk/government/publications/husband-and-wife-civil-partners-divorce-dissolution-and-separation-hs281-self-assessment-helpsheet/hs281-spouses-civil-partners-divorce-dissolution-and-separation-2016
If you and your spouse or civil partner are living together, any transfer of an asset between you is treated as giving rise to neither a gain nor a loss to the person transferring it. Any amount actually paid is ignored. If the person receiving the asset later disposes of it, he or she will be treated as if they had paid an amount equal to the total of your costs.
If you or your spouse or civil partner were living together at some time in a tax year, you can transfer assets between you at any time in that tax year at no gain/no loss. There’s no requirement that you should be living together at the time of transfer.
You need to transfer closer to time you are separate.