Uranium is separated in the gas phase as UF6. (Which is another nasty, toxic and flammable chemical.) The difference between between the molecular weights of the hexafluorides with U235 and U238 is less than 1%, so no matter which way you attempt to
separate the isotopes, it has to have a cascade of many stages and it is is an extremely expensive process.
H2O and H2O2 are totally misable, but the vapor pressure of H2O2 is much lower than H20, so vacuum distillation is the method of choice for separation. You can't use regular distillation at atmospheric pressure since the peroxide will explosively decompose as it nears the boiling point.
Dilute peroxide and
high test peroxide react very differently. Dilute peroxide solutions are used to bleach hair, disinfect water and surfaces just like bleach and performs many other useful functions safely. Dilute peroxide solution will bubble vigorously if a chemically or thermally induced decomposition occurs, but steam will not be generated. High test peroxide on the other hand decomposes into steam and O2, instantly increasing its volume by a factor up to 5,000, and will explosively rupture a sealed container. It has been used to make IEDs and other explosive compounds and therefore is monitored by HSA.
While there there are niche spacecraft applications, hydrogen peroxide is a very mediocre and expensive propellant, and simply isn't worth messing with when compared to APCP or hydrids which are far cheaper, better and safer propellants.
Bob