If you sell the property in July 2014 for £675,000 you will make a gain of £500,000 (£675,000 - £175,000). By that time you will have owned it for 201 months of which you will have lived in it for 60, let it for 136 and it will have been vacant for 5.
The gain for the period the property was your main home will be exempt from CGT as will the gain for the last 18 months of ownership. That accounts for £194,030 (£500,000 / 201 x 78). The remaining taxable gain of £305,970 is that part of the letting period gain which is not covered by the last 18 months of ownership (£500,000 / 201 x 123).
As the property was both your main home and it was let you are entitled to letting relief which is the lesser of:
2 the sum of the main residence gain and the gain for the last 36 months of ownership of the property which is £194,030 and
3 the letting period gain of £305,970.
Letting relief of £40,000 will reduce the remaining taxable gain of £305,970 to £265,970 and the annual CGT exemption of £11,000 will reduce it further to leave you with a net taxable gain of £254,970.
There are two rates of CGT, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of your income in the tax year that you dispose of the flat. As your income is well above the point at which you start to pay income tax at 40%, all the net taxable gain will be charged at 28% do your CGT liability will be around £71,391.60.
You can take account of purchase and sale legal fees, survey fees, stamp duty, selling agent fees and other disbursements when calculating the gain so you should be able to reduce the tax liability a little.
If you are married or in a civil partnership, you could reduce the CGT exposure by putting the property into joint names but in order for such a move to be effective insofar as main residence relief is concerned, the property would need to be your main home at the time of such a switch. Furthermore, the property is no longer let so your spouse or civil partner would not qualify for letting relief as the property would not have been let during their period of ownership. The loss of those two reliefs on half the gain would almost certainly negate the potential tax saving to be had if your spouse or civil partner was a lower rate taxpayer.