The Transfer of Undertakings (Protection of Employment) regulations 2006 (known as TUPE) are designed to protect the employment rights of employees who are affected by a business change, for example when companies merge. The general principle under TUPE is that employees will have the same rights with the new employer. This is not pensions law, it is employment law.
If your employer is taken over or merges with another company or your role is outsourced to another employer, and you remain in employment, the pension benefits you've already built up in a workplace pension scheme are protected. Different rules apply to private and public sector transfers as to what your new employer must provide in the future.