The dirty mrks on my original tracks have been with me since I regrooved with Dave Wilkinson's 67 Cooper laps when I was first learning how to do this lark.
They have to stay!!! Especially at Silverstone where there is precious little else to look at. The regroove at Nurburgring didn't work vert well and snakes about on and off the track but it still has to stay.
When using them for 65s I brake about half way along the dirty mark.
I make do without on my 66 install because I can't be bothered.
But if you could be bothered you can install any graphic update and then regroove again with your preferred lap, eg Dave Wikinson 67 Cooper, and you will have your dirty marks back and the different trees!!
And as I say with some file manipulation and renaming you can have a different groove for every mod. It is a lot of fuss I have to admit to go through the process, and you do have to be careful you don't get mixed up along the way.
I'm sure there is a ready made brake marker update for Silverstone but anyone can do it as mentioned by using the TRL utility.