I can tell you that a one way trip Southampton to Southend by taxi will cost about £140. Try checking with taxis at your end to see if the price is the same or better. It may be far more expensive than coach travel but ship to doorstep can't be beaten for convenience......Nei.
We're braving the train in May as the station is a very short taxi from the port and we get into Waterloo then C2C trains out Shoeburyness way. Cheap and cheerful at around £40 each but maybe the extra £60 would be worthwhile.