I had the same problem on my Escort. I didn’t think much of it to start with but then one day, it got so bad that I managed to drive the 5 miles to work without using the accelerator as the revs were stuck at around 3k and the only way to get the level to drop was to turn the engine off 😐
I jacked the car up and emptied the oil out before removing the idle control valve, cleaning it with petrol & WD40 and then putting it back on and putting new oil in. That was in June and it’s been fine since, although the engine does idle at about 1.5k when it’s cold but it’s an old car so it won’t be perfect.
See Link on how to clean the idle control valve (note – link is for an Escort but the process is the same and the valve for the focus looks the same as the pictures)
It’s not a big job and providing you’ve got a couple of axle stands you’ll be able to get under the car to change the oil and remove the valve. It took me about an hour and half to do it and only cost £1 for petrol and £15 for new oil and a sump plug washer so it’s worth a try before taking it to a garage and being charged an arm and a leg (and other assorted body parts).