As a peer comment says, the risk with this is treating a de facto employee as a contractor.
In the UK we have a law called IR35 to prevent disguised employment. If a US company hired a UK based citizen and acted in breach of IR35 then the employer would be liable.
In reality it would be hard for tax authorities to enforce, but I think it would be enough to scare off the employer.
Nitpick, but I don't think this is necessarily correct - if the end client has _no_ UK presence, then the "old rules" apply for IR35, i.e. the contractor is responsible for determining their own status.
If the end client has some UK presence (a branch, etc), then yes the "new rules" apply and the end client is responsible for determination.
In the UK we have a law called IR35 to prevent disguised employment. If a US company hired a UK based citizen and acted in breach of IR35 then the employer would be liable.
In reality it would be hard for tax authorities to enforce, but I think it would be enough to scare off the employer.