Lord Sinderby must be a Baron, for the way that people call him and the tittle is new, his butler mocked about it, also when Rose spoke about Atticus family for the first time Robert noticed that they basically were the "new kid on the block" and for that reason organized the horse race, in order to imprese the county that they now lived because, and Robert also noted, they basically bought some big house but the local people didnt want them so they were trying to charm everybody there. Also the way how they made their fortune, Atticus said that come from the banking business, the complete oposite to the old money who basically were landed gentry and the way that Lord Sinderby conducted himself, he was very snob and didnt have that "aristocratic charm" and basically he was imitating the way how the aristocracy behaved (like renting Brancaster).
I read somewhere that because England was so broke after the war that they basically sold tittles, so, people with a lot of money and connections finally obtained their tittle and joined the aristocracy. I am only guessing but it is very possible that even Carlisle obtained a tittle also if he pay it for it.