Theft Act 1968
ishonestley = needs to be proved
Appropriate = needs to be proved
Property belonging to another = needs to be proved
With the intention to PERMANENTLY deprive them of it = needs to be proved
Siezing property of similar vale as a lien will not come under the theft act.
However making off without payment i.e. refusing to pay is
If you can show (possibly by reference to previous conduct) that a person or company has ordered work with no intention to pay, this would be a criminal matter.
SteveT