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.
Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!