On the occasion of absolute entitlement
The trustees are treated as disposing of that property at market value. There is therefore potentially an occasion of charge to CGT, depending upon the assets concerned and whether any exemptions or reliefs can be claimed.
The trustees are deemed to have reacquired the property in question at market value as bare trustees. CG34300+ explain how CGT applies where property is held by bare trustees. The basic principle is that the trustees are disregarded. The beneficiary is treated as the owner of the property and anything done by the trustees afterwards is treated as if it had been done by the beneficiary.