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If you give the flat to your son in the current tax year, 2016/17, you will each only get one CGT exemption not seven. The CGT exemption for 2016/17 is £11,100. The value of the flat when it is gifted will be your disposal proceeds, 50% for each of you and your wfe. You deduct from that the cost when you bought it, 50% for each of you and your wife. You can also deduct the costs of purchase (legal fees, stamp duty, survey fees, etc) when calculating the gain, 50% for each of you and your wife. You then deduct £11,100 from your respective shares of the gain to arrive at the bet taxable gain for each of you. Take a look here for more information.
I hope this helps but let me know if you have any further questions.
Each time you transfer a portion of the property, you use the market value of that proportion of the flat at the time of the transfer as your "disposal proceeds". So, your example in your original question is correct. The value of the proportion given away will vary from year to year depending on property price movements.