It's very difficult to transform a string into an array indexing command, which is what you're basically trying to do.
If all else fails, couldn't you just hard code it, since you only have 5 boxes?
Code:
if (variable1 == 1)
variable2 = UVR_txt1.text
else if (variable1 == 2)
variable2 = UVR_txt2.text
else if (variable1 == 3)
variable2 = UVR_txt3.text
else if (variable1 == 4)
variable2 = UVR_txt4.text
else if (variable1 == 5)
variable2 = UVR_txt5.text
Sure, it's not the generic solution, but it works for five textboxes.