I don't shoot motorsports much, and I've only been to Silverstone once, but here is an example on a sunny day with my 40D and 100-400mm lens at 400mm, 1/250, f/10, 200 ISO. This is uncropped but has been edited to add some "pop" since the fencing definitely causes loss of contrast....
If you have to shoot through fencing then some tips that help - use the longest focal length you can to get the framing you need, get as close to the fence as you can and use the largest aperture you can that provides the DOF you need. These steps will all contribute to making the fence as OOF as possible and as inoffensive as you can hope. Also, if you have the sun behind you, try to find a spot that puts the fencing you are shooting through into shade. If the fence is sparking in the sun it will add to the disappointment.
Choosing the right shutter speed is key to getting the right amount of wheel and background blur, without suffering too much shake. Juggling aperture and ISO to get to those desired speeds is the tricky part, especially if the lighting is not working in your favour. On a sunny day, to get a shutter speed of 1/200 at 100 ISO you will need to stop down to f/11. If you have an ND filter, or that CPL, it might help you open up the aperture a bit, which will help lose the fence. If conditions are less bright then you might find yourself needing to open up, which can only be a good thing.
Unfortunately, I guess you are stuck with shooting at 200 ISO, which is going to make things tougher if the light is strong. Pray for some cloud cover.
My personal recommendation is to shoot with manual exposure. That puts you in control of everything. Others may disagree. That is their choice. I won't waste my time arguing that point. Manual exposure will keep white cars looking white, and black cars looking black, and the track and surroundings will be the same tone from one shot to the next. If the light changes then adjust your exposure. That need may arise very infrequently. That's got to be better than fiddling with exposure compensation for each and every car that passes you.
Here's another example at Silverstone. The cars were much closer here, requiring a focal length of only around 200mm to fill the frame. Unfortunately, the combination of closer focusing, shorter focal length and a "silly" aperture (no choice without filters) has meant that the fencing is far too obtrusive. Hopefully you can learn from my mistake