You don't change automatically but you are free to use whichever name you
prefer - simply tell people you are now using your maiden name (if that's
what you want)
For official documents, most places (banks etc)
should be able to change back to your maiden name without needing a change
of name deed if you can provide suitable evidence - typically your birth
certificate and decree
absolute. The passport agency will also make a change
on that basis if you send a covering letter.
(If your bank
won't, then you could change your pasport and then open a new bank account
elsewhere!)
The only people you may have a problem with are
Benefits and Tax offices - but even if they require a change of name deed
you can still start using your original name straight away - as long as you
are not doing it to defraud anyone, you can call yourself whatever you
want..