I don't specifically recall which banks were at CNX. I have a vague recollection of Siam Commercial and maybe TMB, but that may be the power of suggestion based on its presence at BKK. I think I spotted one UOB somewhere in Chiang Mai city, but I couldn't say where. Spotted no foreign banks on the Night Bazaar street.
HSBC has a location in the financial district along Sathorn Road, as does Citibank, ABN Amro (or whatever it has become), Deutsche Bank (IIRC), and many others. But I don't think HSBC has much of a retail presence anywhere in Bangkok. UOB is the one major foreign bank trying to gain a retail foothold, which it did by acquiring someone else (I forget whom). Standard Chartered is a few steps behind UOB in that process. I'm fairly sure I've seen a UOB at Siam Paragon, FWIW.