IR35's Silver Bullet is...
There is 1 silver bullet that kills the IR35 "disguised employee" argument, and that is the Substitution Clause. Here's how to do it in practice:
Just to be clear, being able to substitute yourself in a role is a cast iron "I am not an employee" card, because no employee can choose to send someone else in their stead; case law: Ready Mixed Concrete (South East) Ltd v Minister of Pensions and National Insurance (1968).
However the substitution clause itself isn't enough, it can't be a "sham" clause; case law: Autoclenz v Belcher (2011)
Here's how you prove your substitution clause isn't a sham, when working for a company that requires ID, and logins and passes etc to turn up on-site and work; it all revolves around "working from your home office":
1) First decide to work from home office one day. Of course a courteous contractor would inform the client's Project Manager (PM) about this beforehand, and accommodate any reasonable client requests about when this would be most convenient, but in 15 years as a contractor, I never had a contract where this wasn't possible to arrange, even if for just one day.
2) Agree with the client's PM that you'll be working on documentation whilst working off-site. Ensure that said documentation isn't anything excessively sensitive to the client's IP or security.
3) Agree with the client's PM that in order to get the most effective work done by the most able people, and to help meet the next deadline, you'll get in additional documentation support, in the form of a mate or spouse say (your substitute), doing some graphs, tables, spell checking, formatting etc, of said documentation. All at no extra cost to the client. Allowing you to focus on the core technical work. (It's possible to skip the agreeing with the PM bit if you don't get on with them, they're unlikely to notice.)
4) Ensure that the documentation version history accurately reflects that the document was changed by said substitute, including their name and company (you do make sure you put your own name and company on all document version histories, right?).
5) When billing for this month, submit your invoice as normal, but add an extra line showing the work down by your substitute. Ensure as you've agreed with the PM in (3) that there's no extra cost for this, so you'll likely have an invoice that instead of saying "20x days for me", it says: "19x days for me, 1x day for subbie". The total is the same. Submit this in the usual way.
6) You may need to explain to the client's finance team that the odd extra line was cleared by the client's PM already. Most likely, they won't even notice. If they're an agency that operates "self billing" then they likely won't even look at your invoices anyway, but as a real business, you should always create your own invoices and file them accordingly. Arguably this is even better as there's no one to query what you put on your invoices.
Now you have a cast iron paper trail of actual substitution, with your subbie doing real work, documented in the version history, and their time being invoiced independently. Your arrangement is now incontrovertibly incompatible with one of being an employee. And yes, I have done this, several times. Copies of the invoice and documentation are kept in my "IR35 defence file", you have got one, right?
Tada: one IR35 silver bullet! :D