Oops, the above was for Scotland.
Here's what it says for England:
http://www.adviceguide.org.uk/index/your_rights/legal_system/police_powers.htm#your_rights_on_arrest
How long can the police hold you
The police should not detain you for more than 24 hours without charging you, unless an officer with the rank of superintendent (or above) or a magistrate gives permission.
A police officer with the rank of superintendent (or above) can authorise detention for a further 12 hours. Magistrates can authorise further detentions up to a maximum of 96 hours. Once charged you should be brought before the magistrates the next day (but not on Christmas Day, Good Friday or any Sunday).
But like I said before, if the cells are full and forensics says it was self defence they may decide to release him earlier.