No, whatever you do don't get an RCA to F-connector. That'd be pretty pointless on a UK TV.
On all but the earliest VCS machines in the UK, the connector is a standard aerial lead, you can just bypass the switchbox directly. One very early VCS I have has a phono connector and matching switchbox, the rest are all normal co-ax.
The knob is a pot, yes, (it's short for potentiometer). It may make the picture better because it changes the position of the sound signal relative to the video signal. If your sound signal is too close to your picture signal you'll get interference on both.
How good is your picture quality? If it's poor, you might just need a signal booster (£10-£15 from Argos kind of thing).
Thanks both of you for your replies. Yes, the connector is a standard aerial lead which I connect directly from the Atari to my PAL TV. The picture quality seems ok to me, it's just the sound. I get absolutely no sound, just constant interference. By the way, do you have a picture showing the location of that pot I need to adjust? Would it be possible to upload it here for me to see? Thank you in advance for your time.
Best regards,
Yannis