Option: gift it as is, and niece runs risk of not getting planning, but value is less so less tax.
If he has a good farm accountant, they should be very familiar with how to structure these, and any exemptions that can be applied.
My initial thoughts:
Based on gift valued at 75k:
Uncle will pay Capital gains tax, based on market value minus what he originally acquired for.
There are some exemptions he can apply in certain circumstances. If he's aged 55 or over, retirement relief can be applied. But there is a lifetime limit for him, and if he exceeds it, the exemption is removed.
Uncle to niece is Category B for CAT (gift tax). Gift is in excess of lifetime limit, and ordinarily tax on amount in excess of 32.5k + 3k annual small gift exemption would apply. However, tax treatment is that there isn't tax twice on the same transaction, so if uncle has paid CGT, she can get credit of the amount of CGT uncle pays, which might reduce her tax to zero. But she will have used her lifetime Category B allowance.
As its just land, she pays stamp duty at 7% on the market value on transfer, but can apply for a partial refund once she builds & occupies the house.