Lord

A title for a marquis, earl, viscount, or baron (whether a peer or a peer's eldest son holding his father's second title by courtesy), never used with of:

Lord Hartington, Lord Derby, Lord Manvers, Lord Palmerston may be used instead of

the Marquis of Hartington, the Earl of Derby, Earl Manvers, Viscount Palmerston.

It is prefixed as a courtesy title to the given name, with or without a surname, of a younger son of a duke or marquis:

Lord Randolph Churchill (the third son of the seventh Duke of Marlborough) or Lord Randolph never as Lord Churchill

Lord John Russell.

A baron (whether a peer or a peer's eldest son known by the title of his father's barony) is always called by his title of peerage (either a surname or a territorial designation), preceded by Lord:

Lord Tennyson

If the given name is mentioned, it comes first:

Alfred, Lord Tennyson

Thomas, Lord Fairfax

As part of the titles of certain high officials and dignitaries:

the Lord Mayor of London

the Lord Chancellor

the Lords of the Admiralty.

As a ceremonial title for any bishop or archbishop of the Church of England:

Lord Bishop.

For specific British titles consult a source such as Burke's Peerage or Debrett's Peerage.

Context generally determines when the use of titles such as Sir and Lord are appropriate:

A comment about Paul McCartney's being knighted would probably use Sir Paul,

but a reference to the rock musician might style him simply Paul McCartney.