Suppose you are a licensed plumber working solo and you have a contract with a builder to do the plumbing on a house that is being built. Your job has to be finished by a given date. If you fall ill or are unable to work for any reason, what happens? Will the builder give you an extension of time? Remember that the builder may have other subcontractors lined up to do other jobs right after your
work is completed, and so your
contract may specifically say that extensions of time are not permitted,
and that penalties will be imposed for failure to complete the work on time.
If the contract does not allow you to send in another licensed plumber to do
the work (and pay that plumber as a sub-subcontractor out of the money you get
from the builder), then what?
Another plumber could, in most instances, replace you and do the necessary
work if you are ill. If you are a software developer working on
site on contract, it might be harder to get another programmer to go in
and start writing code from the point that you left off the previous
evening: the learning curve to figure out what is going on might be just
In the US, the Internal Revenue Service has rules about
when someone that a company claims is an independent contractor (and so
the company is neither liable for various employment taxes nor required
to pay employment benefits) is actually a contract employee because
(among other reasons) the work conditions imposed -- come in to our
job site, do what we tell you to do using our computers, etc -- more
closely resemble employment than independent contracts. It seems from
the website that you provide a link to that the UK has similar rules too.
If you don't have the right to send in someone else to do your work
at the client's site (and also satisfy various other conditions that
a truly independent contractor would satisfy), then,
in the eyes of the tax authorities,
you are effectively an employee of your client
regardless of what you or your client or the contract between
you and the client says.