If you sold the property in your own name, the gain would be £180,000. The exempt gain would be £69,000 (£180,000 / 30 years x 11.5 years). You are given the last 18 months as a tax free period regardless of whether you live there in that time or not. The taxable gain is £111,000 and you deduct the annual CGT exemption of £11,100 from that to leave a net taxable gain of £99,900 on which you would pay CGT of £27,972 (28%). At today's tax rates and assuming your income was less than £11,000, the personal allowance level, you would pay CGT at 18% on £32,000 and at £28% on £67,900. The CGT would be £24,772. For every £1 of income you have in excess of £11,000, the amount taxable at 18% would reduce by £1. I cannot see HMRC allowing letting relief.
You would lose £34,500 in main residence relief if you put the property into joint names on a 50:50 ownership basis. Your CGT would be £12,432 (£90,000 - £34,500 - £11,100 x 28%). Your wife would have a net taxable gain of £78,900 (£90,000 - £11,100) and pay £18,892 in CGT (£32,000 @ 18% + £46,900 @ 28%). Overall, you would pay about £3,352 in extra CGT between you.