Yes. Electronic items are permitted in the cabin but they must not be used while the 'fasten seatbelt' signs are on, during take-off and landing.
Remember that you're only entitled to take one item of cabin baggage onto the flight. Your Nintendo DS must fit inside your cabin bag (although you might be required to remove it for separate screening): http://www.dft.gov.uk/transportforyou/airtrave l/airportsecurity