The British Peers and Baronets Project seeks to bring together all persons in (or from) the United Kingdom with hereditary titles, excluding monarchs (who are already compiled under the "English and British Monarchs" Project).
The term "British" in this context is understood to include all titled peers and baronets in the United Kingdom (this includes all of Great Britain -- England, Scotland and Wales -- and Northern Ireland). Basically, if a subject of the British Crown possessed or possesses an hereditary title, ranging anywhere from duke down to baronet, they belong in this project.
Please note that just because a person's name is preceded by "Lord" or "Lady", it is no guarantee that they are a peer or baronet; for example, the children of some peers are styled "Lord..." or "Lady...", regardless of whether they ever inherit a peerage. Nor should "hons" ("the Honourable...") be included since this prefix is merely an honorific courtesy extended to the children of some peers and is not in itself a title. Finally, the only knights ("Sir..." or "Dame...") who should be included are baronets since, while baronetcy is a species of knighthood, it is usually heritable (the exception being the 20th century practice of bestowing life-baronetcies).
Finally, one matter that's potentially complicated, but I've attempted to make less so: when we think of peers and baronets, we normally think of *men*, although there were some women who possessed titles in their own right (perhaps 3% of all titles). However, when a woman is married to a peer or baronet, or is widowed, she possesses a courtesy title equivalent to theirs. She loses her courtesy title if she is divorced or is widowed and remarries a commoner. But instead of trying to examine each woman's profile to determine if she possessed her title independently of her (titled) husband, I think it's simpler just to include the wives and widows of peers and baronets here. It's erring on the side of granting some women courtesies they're not entitled to, but I'd rather do that than exclude some that *are* entitled. It's just my opinion, but unless there's a groundswell of opinion to the contrary, we'll try it that way.
(One confusing thing that's peculiar to British titles: the wife of an earl is called a "countess", but there are no "counts" in British peerage.)
If you are not certain that an individual legitimately possessed a title, do not include them.