IR35 - As I recall ..
was introduced to prevent exactly what was happening when I worked for a GEC company in 1998-2000.
Employee would resign on Friday, and then turn up on Monday doing exactly the same job, but as a "contractor". With an appropriate bump in salary as they then paid themselves in dividends.
It was discussed in great detail on this very site - I recall quite a few people threatening to fuck off to the US if it came to pass. I guess we really missed their like.
From what I recall the IR35 tests were quite sensible at separating employees from genuine contractors supplied by a genuine company. The key one being "substitutability" (?). If your contract with the company allows you to send "a resource" then they are probably genuine. If, for any reason the contract is for a particular person, then it's probably bogus.
Or to put it another way the last 3 building jobs I have had done were done by a bunch of lads who I had no say over - I sorted it with the gaffer, and he chose the resources.