Work aren't paying you to be in the building. They're paying you to do a job and you should be ready to start the job at your start time.
It really surprises me how many adults don't realise this. Especially if you're providing a service. Are customers supposed to wait 5 minutes until you get logged into your till/phone etc? What if a colleague is finishing at your start time? Do they wait until you've stuck your stuff in your locker and sauntered over to the tills?
If they need/want you to be ready to deal with customers/clients at 9, then they need to pay you for your time - not expect a freebie. All they have to do is make your start time (and start paying you from) 8:50 and then tell you that you have to have switched your computer on, taken your coat off, had a wee etc. and be ready to actually start work at 9am. Companies will bleat at that: "But that will cost us a fortune paying every single person for an extra 50 minutes a week!" Tough: if the employee is there working for you, you don't expect them to give you 50 minutes of their own unpaid time every week.
This thread has been very focused on office/teaching jobs so far, but when you look at jobs that take far longer to prepare, it becomes obvious.
Suppose you're a scaffolder (paid by the hour from 9am sharp by a contractor), would you expect them to 'have the scaffolding ready for use' at 9? Even if assembling it and doing all the safety checks take several hours? Try finding a plumber or electrician who has to drive for an hour into a city/to a central depot store for an urgent unusual part that's crucial to the customer's job, after diagnosis, and not expect to be charged for their time. Insisting that you are paying the builder to actually build your wall and that he should expect to be mixing up the mortar in his own time before you start paying him?
As long as people aren't taking the mickey, ten minutes is ten minutes - why should the worker (who is required to be there to do the job) have to pay for it rather than the employer who is paying for you to do the job? We've been conditioned to think what is 'reasonable' as an employee, but why is the employee more unreasonable to want to be paid for their necessary work time than the employer is to just assume and expect ten minutes of free prep work every single day?
Ten isolated minutes isn't that much to argue about, but over 48 weeks of the year, it comes to 40 hours - a whole working week (or more for some). Imagine the (fully justified) fury if your company nonchalantly told you they were only going to pay you for three weeks every February - and then called you in for 'a talk' if you had a problem with that?