As far as I know, no-one is 'a Lord' or 'a Sir', that's just what the misinformed would refer to them as.
If you're an Earl, a Duke or a Baron you'll get addressed 'Lord X' and if you're a Baronet or a Knight you'll be called 'Sir X'. There's an order of precedence of the aristocracy with the ruling Monarch at the top and 'younger sons of Baronets' at the bottom.
There's a very good book published by Debrett - it's called Debrett's correct form. Tells you how to address all titled people including military and government, either face to face or in a letter.
Correct. If you are awarded a Life Peerage, (e.g. buy a title for Labour) you'll become John Smith, Baron of Buckinghamshire, (as an example)
Formal Address is
The Rt Hon The Lord/Lady Buckinghamshire
Referred to is
John, Lord Buckinghamshire
Informal is
Lord Smith
Incorrect is
Lord John Smith as the papers often say.
In Descending order of importance:
Royal Duke (e.g. Duke of Edinburgh)
Duke
Begin: My Lord Duke
Address: His Grace the Duke of Buckinghamshire
Speak to as: Your Grace
Ceremonial, formal, or legal title: The Most High, Noble and Potent Prince His Grace John, Duke of Buckinghamshire
Children: Referred to as Lord Forename Surname & Lady Forename Surname.
Except the oldest child where he can use any title lower than that of Duke that his father has. E.g. If his father was also and Earl (lower than a Duke) he could use that tite.
Marquess
Formal: The Most Honourable The Marquess of Buckinghamshire
Informal: Lord Buckinghamshire', and his wife "Lady Buckinghamshire
Children: Same as Duke
Earl
Formal: Earl of Buckinghamshire
Informal: Lord Buckinghamshire
Children: The Eldest Son can use any unused title (courtesy title) or is styled as Lord X. All other children are referred to as The Honourable Forename Surname or The Lady Forename Surname. (Lady Diana Spencer being an example)
Viscount
Mainly used as a courtsey title for children of Marquesses and Earls.
Formal: The Viscount of Buckinghamshire
Informal: Lord Buckinghamshire
Addressed as: My Lord
All children are referred to as The Honourable Forename Surname
Baron- Heredity
Formal: Baron John of Buckinghamshire
Informal: Lord Buckinghamshire
Children styled as The Honourable Forename Surname
Baron - Life
Formal: Baron John of Buckinghamshire
Informal: Lord Buckinghamshire
Children styled as The Honourable Forename Surname, and continue to be even after the death of the life peer.
Traditionally all Prime ministers get offered a Heredity peerage when they leave the house of commons. Thatcher was the last one to be offered it but turned it down saying she didn't have the money to support it and took a life one instead, (Baroness Thatcher). John Major should be next.
Only 5 heredity peers have been awardard since Willson's government (64), 2 of which were to the Royal Family, 2 Viscounts who both died without heirs and The Earldom of Stockton that was awarded to McMillan upon leaving the commons.
Not quite as simple as that as some people use their last names instead of Buckinghamshire etc etc but that's the rough jist. If you're setting a table then you have to integrate the church and army ranks into that list as well.