Firstly, do not plan to drive in Paris. Apart from the traffic, lack of parking and general hassles, it's an easy city to get around without a car. It's a great walking city, superbly laid out, with flat terrain and close proximity of each of the main attractions.
The Metro (and linked RER) is an amazing rail system with stations every 500m or so. It's incredibly clean, (it kinda puts the London Underground to shame!), efficient, convenient, inexpensive, and very user-friendly. However, all signs are in French so it will pay to brush up on your schooldays French and also to carry a Metro map. Once you get your bearings and master directions as in North, South, East and West, it's a snap!
Taxi drivers can be erratic and somewhat arrogant - but they may well be putting this on to live up to their reputation. Taxi prices are quite reasonable, however it would be wise to have a fair idea of the distance to travel and your destination, (it's a good idea to carry a pocket map at all times), as in any other big city, there are unscrupulous taxi drivers that will drive you around in circles if you look like an easy "mark". Most drivers speak a little (!) English, but don't count on much. Again, a good idea to know some basic French words especially for directions!
Getting from London to Paris is easy and relatively inexpensive. You can fly, drive, hop on a ferry or take the train. We REALLY recommend using the train - it’s 3 hours from London (St Pancras International) to the centre of Paris on the Eurostar express train direct to Gare du Nord in Paris.
Considering the hassles of flying, (security, check-in, baggage collection and expensive airport transfers), the Eurostar is much quicker. You can now also take Eurostar from Ebbsfleet in Kent (via the M2) which has a huge parking lot with excellent parking facilities and prices for short-medium term parking. Eurostar in our opinion is by far the best option for travel from the UK both in convenience, comfort and value.
Passport and security checks are carried out on check-in prior to boarding at both stations, but are relatively quick and hassle free. Departure from London does not usually require you to produce your passport, but you are required to produce it in Paris at check-in for your return to London. Also worth considering, the Eurostar also operates extensive services to the beautiful medieval French city of Lille and Brussels in Belgium with hassle free connections to to many other European cities. Perfect for a romantic weekend getaway!