The purple goose in Woburn does fantastic food, I've only been in the evening but hear good things about their lunch menu.
http://www.thepurplegoose.co.uk
If you fancy something pubbier then the carrington arms in moulsoe is also great
http://www.thepurplegoose.co.uk
Both are only 10-15 mins from the centre of MK and well worth the the drive.
Enjoy your lunch wherever you end up!
Rob