most coach firms try to get on early morning ferry arriving in UK about 8-9 am. dont know your destination but there is bus every 20 mins from dover docks to the station and the train will start from there.
because of possible delays at customs/immigration i dont suppose anyone could give you an accurate enough time to book a seat on a train.
have a look at this site for info on trains
http://www.nationalrail.co.uk/