I prefer to write them "Upsadaisy" and "Whoopsadaisy" and in both cases the first vowel is pronounced, not like the "u" in "but" and the "oo" in "boot," but (both of them) like the "u" in "put."
It's still an occasional part of my active vocabulary, e.g., directed at myself when minor mishaps occur or possibly, ironically, more major ones.
I grew up in the southeast of England, but I'd be wary of tying my acquiring it and using it to the region.
OED lists two spellings, two distinct entries:
1. Used to express encouragement or reassurance, typically when assisting someone (in early use esp. a child) to get up, recover from a fall, etc., or when lifting a child into the air. Cf. up a daisy int.
2. Used to express surprise, concern, etc., typically used in response to a minor accident or mistake. Occasionally as a modifier, designating such a minor mishap. Cf. whoops-a-daisy int.
Used to express surprise, concern, etc., typically in response to a minor accident or mistake. Sometimes also used when encouraging or assisting a person to perform a simple physical task, esp. one regarded as difficult or daunting (cf. upsadaisy int.).
---the OED pronunciations (particularly at the second) are many. ("My" pronunciation of "upsadaisy" isn't listed, which likely won't, I fear, succeed in making me change my ways. ;-()