Assuming you have never used that house as your primary residence, your Capital gains would be = Net gains from the sale - CGT allowance - Expenses (like stamp duty, legal fees on purchase/sale, Fees of estate agent, Property improvement costs)
Net gains from sale = (£600,000 - £120,000) = £480,000
CGT Allowance (for 2017/18)= £11300
Expenses = Depends upon how much you paid
Capital Gains liable for tax = £480,000 - £11300 - Expenses = £468,700 - Expenses
For more accurate calculations, I recommend that you use the following calculator and enter all the personal information requested -
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