Hello, i am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
You will be entitled to Private Residence Relief (PRR) for the time you lived there plus the last 18 months of ownership as you are deemed to be in occupation even if this is not the case. Actually the calculation for CGT is worked out in months, but I will be able to give you a rough steer.
Your total ownership time is 29 years and your let period 15.5 years. Thus 15.5 / 29 [say 54%] of your gain on sale is exposed to the tax. Gain is 700K - 150K = 600K, at 54% is 320K. Now deduct 11.1K non cumulative Annual Exempt Amount (AEA) and 40K Lettings Relief (LR) leaves say 270K taxable. this is taxed at 18% or 28% or a combination of the two rates depending on your income including the gain in the tax year of sale. A worst case scenario is a bill of some 75.5K.
Your question said we. if this is jointly owned with your spouse then the gain is split half each 135K per head. Both receive the AEA and LR so the bill might be substantially reduced to say 47K worst case between the two of you.
I do hope that you have found this quick canter through CGT of assistance. By the way, any gain is net of purchase costs, selling costs and any improvements eg installation of double glazing, central heating extensions etc but not routine maintenance which can be set against rental income for the let period as can the interest element of any mortgage. Confused, you will be!