Not only are you not allowed onto the platform, they don't even list the platform number on the departure screens until 10-15 minutes before departure. So there is no real advantage in being early.
They say "doors may close up to 30 seconds before departure", but that is just to cover themselves.
Allow 5-10 minutes to walk in off the street, or up from the tube. I would aim to get to the station 30 minutes before departure.
It is a fairly simple station, open plan, one big concourse and all the platforms on the same level as the concourse. You can't get lost.
It all depends on where you are coming from, by what method of transport, what contingency you want to allow, and if you want to buy food and drink at the station.