I know that you can get a lot of this information from Google search, but I wrote this for another Regency RP (no longer active) and I hated seeing it go to waste. It's a little bit simplified, but it should help you find the title of peers and their children in most cases.

Noble titles were everything in Regency England. Marrying for a title was almost as good as marrying for money. Here is a list of British noble titles and forms of address, in order by rank.

Also note that females cannot inherit titles beyond courtesy titles. Even the only surviving daughter of a Duke would only be known by a courtesy title as "Lady Elizabeth" unless she married a peer.

Peers

Duke
---Oral address: Your Grace
---Female form: Duchess
---Female form of address: Your Grace

Courtesy titles for the children of a Duke:
the heir apparent: Marquess
younger sons: Lord
daughters: Lady

Marquess or Marquis (two different spellings, the second is French)
---Oral address: My Lord
---Female form: Marchioness
---Female form of address: My Lady

Courtesy titles for the children of a Marquess:
the heir apparent: Earl
younger sons: Lord
daughters: Lady

Earl
---Oral address: My Lord
---Female form: Countess
---Female form of address: My Lady

Courtesy titles for the children of an Earl:
the heir apparent: Viscount
younger sons: The Honorable
daughters: Lady

Viscount
---Oral address: My Lord
---Female form: Viscountess
---Female form of address: My Lady

Courtesy titles for the children of a Viscount:
the heir apparent: Baron
younger sons: The Honorable
daughters: The Honorable

Baron
---Oral address: My Lord
---Female form: Baroness
---Female form of address: My Lady

Courtesy titles for the children of a Baron:
the heir apparent: Lord
younger sons: The Honorable
daughters: The Honorable

Gentry
There are no courtesy titles for children of gentry.

Baronet - this is a bought title, it is essentially an inheritable knighthood.
---Oral address: Sir
---Female form: Lady
---Female form of address: Lady

Knight
---Oral address: Sir
---Female form: Lady
---Female form of address: Lady

Squire
A squire is not really a title, but this is often used to describe untitled gentry, usually having ties to the peerage. There are no forms of address.

Dowagers
Dowagers are the widows of deceased peers. After her husband's death, the widow is know as, for instance, "The Dowager Duchess of ___" so as not to be confused with her daughter in law.

If a dowager remarries, she keeps the title, even if her new husband is a commoner. For example, a party invitation could read:


in which case the two are understood to be married. However, if her new husband holds a higher title, such as 'Duke', she will probably stop being addressed by her old title.