Not a good idea to take a taxi!
Hope you're not just posting that to advertise for that taxi company because you would need to have at least 6 people to put in the taxi, which obviously is not going to happen. They are the most expensive way to get from Dover to London, so it's only for those who have alot of money to spend. Taxis run about $200 plus gratuity (figured another $20 - $30) one way whereas a train takes less time and only costs $35 per person. You can also do a bus or the cruise line's transfers for less money than a taxi.