Canadian in us for on-site job

Thanks for your answer !

Your experiences are typical.
You have been admitted -- and will continue to be admitted -- on B1 business visitor status (although you are not handed any status document because you are cdn).

I always have any vendor/contractors coming down carry a letter which states what/wher/when they are doing (since it is B1-allowed work), their citizenship, and the fact that they will at all times be employed and paid by the Cdn (in your case UK) company. have the US client write the letter.

Then, the CBP officer will either assk to see the letter, or in 80% of cases, merely wave you through.

How hard is it to get back into Canada after the job?