I work for the company that supplies the VOSA dynamic weighbridges and there are guidelines issued regarding overweight limits.
Briefly they are as follows:
Below 5% - verbal warning and possibly an advisory letter to secure better adherance to the law
5% - 10% A prohibition notice in all cases unless in exceptional circumstances i.e hazardous load. Prosecution should be considered where a prohibition may be insufficient to ensure adherance to the law, usually if the operator is a persistent offender.
10% or more legal proceedings to be taken against driver and operator in all cases with the most significant overload being prosecuted.
There are other applicable details but those are the main ones.
As in all cases though, if you get pulled and greet VOSA with a torrent of abuse, they’re likely to park you up and find something wrong even if you’re not overloaded.