Charge for trespassing?

[speedking31]

I've read many times on here that you can't stick a sign on your drive saying that if anyone parks there you will charge them £100 for parking, and then enforce the charge. If someone does park on your drive and causes no damage, just an inconvenience, then there's little you can do.

In that case, is this sign enforceable? On a building site in Manchester. Genuine pre-estimate of loss?