Examiners should initially assume that all features of the independent claims are essential.
"An applicant for a patent will want his leading claim to be as wide as is possible consistently with its validity. He will wish to avoid any unnecessary restriction of it. So he will want to avoid claiming any feature which would unnecessarily restrict the scope of the claim. A well drawn leading claim will accordingly specify only those features which are essential to the invention claimed. It follows, in my opinion, that the applicant probably regards all the features of his leading claim as essential features of the invention."
Catnic Components v Hill & Smith Ltd (1982) RPC 183 at page 227.
However, examiners should also note 18.104.22.168 Mere Presence in Claim Does Not Ensure Essential.