Why do you want to sell and buy again?. You incur agents expenses and tax liability for both transactions, why not keep the existing flat collect rent until you know your future situation, if you own it when you die CGT is deleted and it forms part of your estate and you get the Inheritance Tax allowance
You will need to pay tax on any “Capital Gain” that is increase in value. How much depends on the housing market in your area, the housing market in many areas has been pretty slow in many areas and I don’t think anyone is expecting a boom soon.
You don’t need to get professional CGT help until you actually sell it, if you make a profit great but generally property sells for less than you expect. Where CGT really hurts is when you have owned property for 20 yrs plus and there is a large increase in value.