No, it is the real world! I remember school fees for my daughter in the 80s, now a Doctor of Engineering, living and working for the government in New Zealand.
I can see why you did not elect, but as you did not HMRC may well base their decision on the facts and as you lived on an off in this second home then they may insist on PRR being applied thereto. In the long term that could leave you with a huge CGT bill when you sell out in London.
The Stamp Duty reduces your gain to 114.5K, 57.25K each less the AEA = 46.15K.
For your husband add 6K to 46.15 = 42.15K. 31865 - 6K = 25865 so that would be taxed at 18% = say 4656 plus 10285 @ 28% = say 2880 so total CGT for him would be of the order of say 7536. Better than the proverbial poke in the eye with a sharp stick.
For you the whole gain will be taxed at 28%, 14.81K.
So between you the total bill aggregates to say 22.4K. However, as you occupied after your return you can claim Lettings Relief (LR) up to 40K so you may find the whole bill extinguished all together.
In any event, if HMRC decide that it is the residence to which PRR applies, it seems to me that with the time in Singapore, the last 18 months or ownership and your occupation time that PRR will apply throughout and CGT knocked out altogether. It all depends upon which way the HMRC cat jumps.
I fear that you may need a local, trusted professional to guide you through the negotiations with HMRC. This is an extremely complex situation.