I think it would be an useful feature to support. Although I believe you need to handle both sending and receiving the shares, it's likely both people will be using the tool.

1. Sending should be easy to implement as it just reduces the 104, although more careful consideration might be needed when the other rules come into scope. It might be helpful to report the value of the gift too for the reason below.
2. Receiving is harder to implement, because the giftee needs to somehow input the value of the shares when they were first owned by the gifter.

I don't have transaction of these sorts so any help for implementing this would be appreciated.

Related: https://www.gov.uk/capital-gains-tax/gifts

I was looking into that and came to the same conclusions. Transfering to spouse should not be a subject to matching rules, only reduces the Section 104 (based on https://community.hmrc.gov.uk/customerforums/cgt/cd55f761-e320-ef11-a81c-0022481b5e57). I have created a patch for that. I have such a transaction but only use the raw format so haven't added it for other broakers.

Regarding receiving shares I am not fully clear whether it is subject to the same day and B&B rules. Some time ago I saw a post on the forum which implied it is, but when I checked again now that post is gone, so maybe it was wrong. Otherwise getting the quantity and value will be easy as it will be the same as the quantity and value that was removed from partner's Section 104.

Transfering to spouse should not be a subject to matching rules, only reduces the Section 104 (based on https://community.hmrc.gov.uk/customerforums/cgt/cd55f761-e320-ef11-a81c-0022481b5e57)

Are you 100% sure about that?

In another popular tool to compute CGT they say the following about transfers to spouse:

https://www.cgtcalculator.com/example7.htm

When your shares are transferred to a spouse the shares are matched up in the same way as if the shares were sold. Your spouse acquires your shares for the same amount as the original acquisition cost. The result is zero gain in your capital gains calculations. For more see this post.

In the above link, which leads to ADVFN forum, they argue that the transfer to spouse is still a "disposal" even though a special one where there's no gain nor loss:

you need to identify the shares that you gave to your wife using the share-matching rules that were in effect on the date you gave them to her. Once you know which shares they were, you can work out what allowable costs you had on them, just as you would for any other disposal. Deal with them in your own CGT calculations as though you had sold them for the total of those costs on the date of the gift, which won't realise either a gain or a loss but does affect which shares you are left with for subsequent CGT calculations. And deal with them in your wife's CGT calculations as though she had bought them for the same amount on the same date.

(Note I'm neither a financial nor tax advisor, i'm just a user like you interested in understanding how this works)

I am not financial or tax advisor too, but this answer is from offical HMRC forum and it looks like it was answered by someone from HMRC:

So just to clarify, a transfer to married spouse is also considered disposals and is a subject to "Same day" and "Bed & Breakfast" matching rules, is it correct?

At the time of transfer no. It is only if/when the spouse then decides to sell that it is a disposal.

I agree this contradicts the example you linked in https://www.cgtcalculator.com/example7.htm. Wondering if this information is verified by a tax advisor or someone from HMRC. I think the post in ADVFN could also be written by anyone.

I believe another reason why treating it as a disposal may not make sense is in the situation where you dispose and sell shares in the same day. Based on the CG51560 you should treat that as a single transaction:

All shares of the same class in the same company disposed of by the same person on the same day and in the same capacity are
also treated as though they were disposed of by a single transaction, TCGA92/S105 (1)(a).

But you can't pool them together into a single transaction because part of it creates chargeable gain and another not. Which complicates further same day and B&B matching. Here is another question about this in HMRC forum: https://community.hmrc.gov.uk/customerforums/cgt/74839f45-ef0a-ef11-a81c-002248c8d5a8 but they didn't provide a clear answer. I think this problem also may be the reason why the cgtcalculator tool doesn't allow this case, from https://www.cgtcalculator.com/example7.htm:

There is a minor restriction with this calculator when using this function. No buy or sell trades with the transferred stock can be
performed in the same date as the transfer.

But nothing prevents me from doing such a transaction in the real world so it must be possible to calculate it.

But as I mentioned I don't work in this area and I cannot be 100% sure. So to be safe it may be a good idea to double check with a tax advisor.