Make sure it is a joint account and yes it goes to the survivor outside the will
the jointure can be converted to a commonalty by a letter or document - so long as the document is NOT a will ( see above)
The moolah is part of the estate for IHT purposes
On death of one party the other party just tells the bank to put it in one name on production of the death certificate ( I think)
Otherwise you have to freeze the dead persons accounts and the executor does all the fol-de-rol
Either party can dip into the account for any amount
BUT
the leading case is re Northall
http://www.fridaysmove.com/joint-bank-accounts-%E2%80%93-recent-case-re-northall-deceased-2010/15832
You cant use it to escape IHT. Mrs N put in £50k and the judges held it was hers and not her grasping nephew's for tax purposes - and is consistent with the law principle of 'equitable tracing'.
This would have been better off in Law - you would not have got so many replies which can only be called 'wrong'