Re: "...GPS blockers as notoriously used by so many professional drivers..."
"Imagine a Speed Camera like device, but fitted with a moderately high gain L-band antenna aimed down the road."
I don't think OFCOM have the money or power do that. I'm not even sure anything illegal is happening by using the jammer unless the user is driving a stolen car. It depends on the band in use and the power of the jammers transmitter. I think the OP is talking about people driving company owned vehicles and not being happy that their every move is being tracked. It might cost them their job if caught, but it's not a fine or prison sentence.