We have a fair number of contractors, who work through umbrellas. that's the only way we accept them and it must be one of our agreed umbrellas - many have small ltd companies too.
Some have been here for 3-5 years on renewable contracts that are usually 6 months at a time. The primary reason we use contractors is to keep flexibility in our headcount. That never seems to change much when the tax rules change and I imagine that if they change again then these guys will still contract. A number of them like the flexibility, some do it to avoid moonlighting and IP issues they would have if they were a perm.
I imagine there will be some groans if their tax goes up but it won't change much about our arrangement with them. This setup works for everyone and tax is a bit of a side issue.
If we are forced to take them on as perms, they'll get the boot instead which will effectively reduce our headcount and make life more difficult for the rest of us. To put it in perspective, we are talking ~40 contractors on a site of 2000 people so 2% of the workforce that we can easily cut if work starts to drop. In the past, we have had fewer contractors and had periodic rounds of redundancy.