Hi. I have a different answer.
Where one of a group of same settlor trusts which were created after 6 June 1978 comes to an end, the annual trust CGT exemption is divided between the number of the trusts in existence for all or part of the relevant tax year regardless of the fact that one of them ended in that tax year, subject to a minimum.
For the next tax year, as the number of trusts in existence is reduced, then the annual trust CGT exemption is divided by the reduced number of trusts in existence for all or part of that tax year, subject to a minimum. See box 5.8 on page TC2 of the trust tax return capital gains pages here. So, you are correct in thinking that the CGT exemption for each remaining trust should increase. The trust CG pages notes are here.
I hope this helps but let me know if you have any further questions.
I stand by my answer. The previous expert's answer contradicted what you already knew, ie that the CGT exemption is restricted where multiple trusts have been set up by the same settlor. You wanted to know the effect that one of those trusts ending may have on the CGT exemption for the remaining trusts.
I confirmed that for the tax year in which multiple trusts exist (5 in your case) the exemption is divided amongst them subject to a minimum and that where a trust is ended, the CGT exemption for the next tax year is divided between the remaining trusts (4 in your case assuming one trust ended in the previous tax year), subject to a minimum.
Let me know if you need further clarification.