Driving from the Netherlands to the Channel
The best route is to drive to Antwerpen, and from there to take the motorway to Gent. There is a sign to drive to Brugge from Antwerpen, but this is not the fastest road. Directly after Gent, you'll cross the motorway Brussel - Oostende, where you turn right, in the direction of Oostende. Before reaching Oostende, after passing Brugge, the road divides into one in the direction of Oostende (straight on), and one to Veurne (slightly to the left). Take the road to Veurne, it will bring you straight to Duinkerken and Calais.
Prices of petrol in the UK are not really reasonable (112pence a litre (Arpil 2008)), only diesel is expensive (114).
Some 20 kilometers after the French border, fifty before getting on the train, you'll find a BP gas station (very small, open 24h); this is the last I have seen so far, and sells diesel. For cheaper fuel, skip the exit to the harbour, and look for EuroCity, a large shopping center, with a self-service, pay-by-card petrol station below the parking level.
Crossing the Channel
If you decide to come by car, I would advise you to take Euro Tunnel (formerly known as 'Le Shuttle'), a very fast, comfortable though not so cheap service.
Arriving at the gate (tolls), how fast you can get on the train depends a bit on the traffic ahead of you, but sometimes you can get on almost immediately.
The ride itself takes 35 minutes, so it is possible to cross the Channel in an hour.
The frequency varies, dropping to one per hour at night, and boarding starts half an hour before departure, so you can get stuck on the parking space for about an hour.
Of course there are also ferries, that are on average cheaper, but take longer; I use Seafrance a lot, but also P&O have a good service; you can book on the internet.
On the motor-way
The best road to take from Folkestone to London to get to my place is by driving towards Junction 4 on the M25, and from there in the direction of Orpington. Be aware that the M20 forks after Maidstone, into a motor-way M26 to "the West", and the M20 to "the North". Although you will be travelling West eventually, you should continue in the direction of the North. When reaching the M25, you just turn left, in the direction of Gatwick Airport or the West. The very next exit (junction 4), after a few miles, takes you on the A21, into London. It's about 80 kilometers from Folkestone to Junction 4.
In the city
From there, the route through London follows the A1, the A232, and the A215.
The distance from Junction 4 to my house is about 30 kilometers, and it depends very much on the time of day how much time it takes to cover the distance.
Clicking on the image below will bring up a screen with a more detailed map (total size 314K):
