I've been trying to get an answer to the same question. I've checked several bank sites, the ICICI have an ATM at Kovalam Junction, so I'm hoping that's somewhere near Kovalam, otherwise it will be a taxi ride to Trivandrum.

Having now returned from Kerela,if anybody else needs to know there is a machine at Kovalam Junction which is a very short taxi/rickshaw/walk from the beach areas. There are plentiful machines in Cochin and we also saw one in Munnar. Changing cash sterling is easy, slightly better rates at exchange shops rather than hotels. Cash avoids incurring multiple withdrawel charges at machines !
On the subject of how much do you need, you can eat well for less than £10 per head including beers. Wines and spirits are more expensive.