Short answer: you will probably be breaking the law and your honest employees may be annoyed when they find out you filmed them.
In the UK, any sort of data collection, particularly video or audio surveillance, is strictly regulated. The Information Commissioner's Office has more information.
In almost all circumstances in which employer surveillance is legal, you are required to notify the employees beforehand of details like the nature of the surveillance, the purpose of the surveillance, and how it will be used.
Just because you have CCTV installed upstairs doesn't mean you are allowed to film them in any other way and for any other purpose than to deter bad behaviour from drunk patrons. Moreover, the expectation is with CCTV is that the cameras are clearly visible.
If the office is accessible to them, your employees might have a reason to go there e.g. for a break from the noise or because it is a private place to make a personal phone call or discuss a sensitive matter with a colleague. They may have an expectation of privacy, especially if that is the only room not covered by CCTV, and it is potentially a breach of trust for you to film them there, even if you do ultimately determine that secret filming is necessary. (They might be fine with it, but you can't know without asking).
You should definitely consult a solicitor before deciding to set up hidden cameras; bring your CCTV paperwork, data protection policies, contracts of employment, etc. if you think you are an exception. You need to be very sure about this, because you could lose your CCTV license, and if the licensing authority requires a CCTV installation in your case, you could also then lose your license to sell alcohol.
I am almost 100% sure that one of my bar supervisors is stealing money
but I have no proof of it because the cameras are just upstairs
You ought to think carefully a) what first made you suspicious; b) what evidence you then gathered which makes you so sure, and c) why it doesn't amount to grounds for disciplinary action on its own.
If you're even considering doing this, you should first think about what you can do to confirm that a) money is missing; b) at what point the money is/has been going missing; c) which employees were responsible for the money on each occasion that money went missing; d) there are no other explanations for the money going missing (Remember, explanations may include such things as a staff member who doesn't know how to void till mistakes.)
When you have a list of all the steps you can think of that would identify the cause of the money missing - including talking to your employees individually - along with results or (if you decide against a particular approach), the reason that step is inappropriate, bring this to the solicitor as well.
Finally, don't assume that a camera will provide you with evidence. You're likely to get videos of your staff entering, using the till, and leaving - whether they're guilty or not. It sounds like you have a system where money can go missing and you can't account for how, and that's a problem a secret camera won't solve.