TUPE protects employees only when the identity of their employer changes. In an asset sale of the business as a going concern, your employees usually move to the buyer on their existing terms. In a share sale the company stays their employer.
Long enough before the transfer to allow consultation, tell the employees' representatives that the transfer is happening, when and why, what it means for them, and what changes you and the buyer plan. Consult them about any changes you plan yourself. Give the buyer written employee liability information at least 28 days before the transfer, or as soon as reasonably practicable after that if special circumstances prevent it.
TUPE business transfers
Your duties as the outgoing employer when employees transfer under TUPE