Most posters here are missing the point. Yes, the trailer tends to block the layby anyway, preventing you from parking there, but that is a positive side effect. No, you can't have stupid signs saying "CCTV operates in this area" because if you are 20 miles from the nearest town, and there is no street lighting, where are you going to put the cameras (and retrieve the images).
These thefts are occurring at laybys on A roads in out of town areas, like the A43 between the M40 and Northampton. I have seen the trailer in question, and it is useful in that I now know something I wouldn't otherwise have known.
You seem to underestimate the word "theft" here. It's not a bloody handbag or a laptop being nicked and a smashed window. It's hundreds of thousands of pounds worth of gear being nicked at once, and the driver is either drugged (gas) or beaten up. The curtains get slashed, and usually the diesel gets nicked too. Lorry drivers have to park overnight, so the length of time for such crime to occur is much longer, and in the quietest hours of the night. You may have noticed lorries parked with one of the back doors open - this is so that the thieves can see what's in the trailer without slashing the curtains, and see that the load isn't worth nicking. The idiots would rather slash the curtains than open the back door you see...
And you posters advocating that we look after ourselves, I just wonder what you would do when faced with 10 or 15 armed men who have decided they want the contents of your lorry. Personally, I'd let them take it - it's not my property, and it's not worth dying for.
Of course, if you would prefer that police patrol every layby on the UK road network at night, rather than concentrating their efforts where the majority of people live, then fine. But try selling that to the general public.
Maybe if the constant decline in secure lorry parks was to be halted, and a few new ones opened up, then we wouldn't have to park on the public roads in the first place. As things stand, a cheap trailer with custom sign writing is a pretty good way of alerting the many drivers who need to know the local conditions.
And yes, I drive artics for a living, and am out from monday to friday so this does affect me. There are many things the general public don't know about the job and the laws concerning driving lorries. The main point is that you can only drive a maximum of 9 hours a day (10 hours twice a week) and you must have 11 hours rest in 24 hours (reduced to 9 hours 3 times a week). So if you have only got 5 minutes left of driving time, it is not an option to "go somewhere else". Or if you have been on duty for 14 hours 45 minutes the same applies. Do you really think we "like" going at 55mph ? All goods vehicles over 3.5 tonnes are restricted to 55mph by law, and this is enforced by mechanical means, so I couldn't break the law if I wanted to (except downhill with a heavy load on).
Remember the old saying - you have to walk a mile in a mans shoes before you can understand him. I'm sure your reaction to this initiative would be different if your car was fitted with a tachograph, and you faced with a £1000 fine if you drove over the limits set by law. Imagine being 10 minutes from home but being forced to sleep in a layby because your time had run out. In fact, you may not have to imagine for too much longer, as I believe cars will be fitted with tachos in the not too distant future. Try getting away with speeding then !