In the UK IR35 is a can of worms - there is no real specific determination of what makes you self -employed - just a balance of probabilities
You should err on the safe side of
1. having multiple employers over a two year period
2. providing your own kit and training
3. not having an office or email or business cards from the employing company
4. having the "right of substitution" i.e you can send someone in your place
5. Determining what hours you work and where and when
6. having your own office and website
7. Not having a roll-over provision in the contract
8. Having a drop date in your contract and make that less than 12 months
9. not attending company functions
Even if you do all of the above you are not completely safe but it makes you a tougher target compared to others...........