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.
47 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!