you can't use chemical dyes on your hair once it's had henna on it. I used henna on mine (which is mousey brown) and it didn't go as red as i wanted. I waited four months and then tried to take it down to dark brown and ended up having to crop my hair (and I do mean crop) because it absolutely ruined my hair. As far as I'm aware trying to strip the pigment out will be even harder as it's peroxide that really reacts badly with henna. Sorry.
I dyed over Lush red henna with L'oreal Casting with good results but it was also a red shade and my hair was in REALLY good condition...you won't get back to blonde without chopping it off though, sorry!
I loved Posh's really short hair, you could have it like that but blonde.