How to Understand the Cooling-off Periods in Contracts?

Is it legal for a company in the United Kingdom to include a clause that basically states that the time I had time to read the contract out of the office of business and in the presence of one of its employees then you waives his right to a period of reflection?

There is no general right to a cooling off period in a contract.  

The point is that a contract is an agreement that is binding once made​​. If you want time to think then refuse to sign it until you are satisfied.

There are some exceptions in certain consumer contracts. 
 
For example, consumer credit agreements, agreements for sale at a distance, and agreements made ​​by sellers calling at their home.

Also

A lease may be annulled if one of the parties who signed under duress. It is unclear what has happened here, but why else does not simply walk away if they were not happy?