Don't bother. You can get a usb charger for Braun toothbrushes for less than the p&p on a psw inverter, and it uses much less power.
I don't think that Braun do one, I know that Phillips make a USB toothbrush but they don't make USB chargers for their normal brushes, I got a third party USB charger for my Sonicare and it works okay, only thing I can say is if the brush is flat it takes ages for it to fully charge it but what I do is put it on the USB charger after each use and it keeps it fully charged ready for the next time.I wasn't aware of the USB charger, but I have a Braun Oral B brush which operates from AA batteries (alkaline or rechargeable). It's OK but not as good as the normal one.
Just had a quick search, the only USB chargers I can find are 3rd party ones, can't find any official Braun ones, still looking,,,,,,,,
My dental hygienist always comments how clean my teeth are. Electric toothbrushes simply do a better job.