Working on a Tier 4 Visa

If you have an international department, ask them.

They're only going to pay me for 20 hours of work, but at a 40-hour per week salary. So it might look a bit odd for someone to be paid £17 an hour for 20 hours of administrative work...

Sorry but this is starting to sound like both you and your employer are trying to get around the immigration rules, and to cover up illegal working.  I think giving that impression is very unwise.

On UKCISA, it claims that your tutor can agree to allow a period of time to be a holiday and must confirm this in writing. However, I don't know where a letter a tutor writes would be sent to allow me to work 40 hours a week, nor have I found anywhere that spells out this process.

You don't send a letter anywhere.  It is a matter of working within the student rules, and ensuring that you are not working illegally.  So if your course ends earlier than planned, or if you are granted a vacation outside the normal university calendar, and hence you can legitimately work more than 20 hours a week, it is a good idea to have evidence that it is not term-time, in case you are ever questioned on it.

If your course ends earlier than planned, you should not be working more than4 months more.

Thanks for all that info. I had completely forgot about the course ending earlier stipulation.

While the BF thinks I should just go ahead since it's unlikely I'll get found out (easy for a brit to say), I'm thinking I'll just have to reject the job. Really depressed about the situation, but I suppose being deported or having issues with future visas would be worse.

Would it be impossible to work part time until you receive your degree, and apply for T1 Post-Study work visa?

Would it be impossible to work part time until you receive your degree, and apply for T1 Post-Study work visa?

The job I was hoping to take was only a three month, full-time position that would end shortly after my course (the dissertation hand-in date). So while I intend to go for a T1 Post-Study visa after my degree, it won't help the job I was offered. I am currently working part time at my university library and also at a museum (still under 20 hours combined), I had just hoped to be able to take a much better job that was in the industry I'm hoping to work in. Plus, the museum work is dreadfully boring!

The job I was hoping to take was only a three month, full-time position that would end shortly after my course (the dissertation hand-in date). 

Oh, right.  Forgot that bit! Sorry!  Best of luck. Hopefully they'll keep you in mind for after you complete your dissertation.