The Fish and Chip Shop is absolutely brilliant. I'm from the North so very critical about London's fish and chips and I think that place is definitely the best yet. Downsides are that you will almost certainly have to book, the coffee is crap and it's really expensive.
These two are great:
Two Brothers in Finchley http://www.twobrothers.co.uk/
Upper Street http://www.thefishandchipshop.uk.com/