That route should be fine although the Thameslink route is closed most nights and weekends. You can put via City Thameslink in the journey planner to force it to give you that route, though it allows far too much time for the short walk from King's Cross to St Pancras, and the National Rail site stupidly puts you on slow trains from Cambridge. If you're travelling on a weekday, get the xx15 or xx45 fast train from Cambridge, and then there are a good number of trains from St Pancras. If you get the xx45 from Cambridge then you should have loads of time for the xx48 from St Pancras (which is the quickest to Sutton). The trains from Victoria are faster (only 32 minutes or so) which is why the journey planner may put you on those trains as opposed to the slower trains from St Pancras.