Travelling to the US for paid business (UK citizens) - Is a visa required or is an ESTA sufficient

The kind of purposes that are allowed for a VWP visit are exactly the same as are allowed under a B-1/B-2 visa, so there are only two possibilites:

• Either using the VWP is fine,
• or the consultants need to get work visas, in practice a H-1B or L visa. These generally require that there's an US employer sponsoring the application.

Whether the work the consultants will be doing is a permitted "business-visitor" activity is not easy to say based on your description. Broadly speaking, the distinction is between whether what they do is in support of the foreign company's economic activity outside the US (such as sales, participating in meetings to coordinate joint activity, or helping install equipment bought from the foreign company, or train local personnel in its use), or it is separately productive work in the US (which earns money independently of anything that happens back in the UK). The latter is not allowed. But there are no crisp rules to tell you which side of the line you're on -- instead there's several decades of precedent from courts and administrative boards, which is not readily summarized.

In any case, if you're the employer and being denied entry would have serious economic consequences for your company (such as losing large customers or defaulting on a contract), you ought to spring for tailored advice by an actually qualified immigration lawyer, rather than what random people on the internet say.

I am a US Citizen and have traveled in a similar role, not to the UK, so my scenario might be slightly different than yours but having worked/got paid in a similar scenario I can provide relevant facts. I used to travel as a consultant.

First I would recommend that you go through the VWP (Visa Waiver Program) for the UK. Link : https://uk.usembassy.gov/visas/visa-waiver-program/

The role the consultants are traveling in to the USA does fall under the category of business provided the following conditions are met

• Duration of stay is less than 90 days
• Must have a valid e-passport. The machine readable passport won't work with the VWP
• Shouldn't have overstayed on the VWP previously
• Must have a return ticket back to the UK or proof of legal permanent residence if they are going to some other country afterwards
• Must have received authorization from the ESTA

All of this is assuming that the consultants are not employees of the client or any other US company, they won't get paid by the US company. They will be paid by their employer back in the UK for this business visit. They must declare money above $10K while traveling.