If your company is taken over by another owner, the law protects many of your rights. When ownership of the business transfers, your terms of employment automatically transfer as well.
It should work as if your contract of employment had been agreed with the new owner when you started the job, although this doesn’t extend to pensions. Your rights come from the Transfer of Undertakings (Protection of Employment) Regulations 2006, which is commonly known as TUPE.
TUPE is extremely complex and time limits in the employment tribunal are very short. If your employer is threatening to downgrade your contract terms after a TUPE transfer, you should take legal advice as soon as possible.
If you’re a union member, you should consult your union so that a collective response and/or legal action can be considered.