1) are there any fines you must pay for failing to inform them that
you moved? What are the consequences of this?
As far as I am aware from my own experiences (Plan 1, income contingent loan), no. There are no fines or penalties for missing payments or being late to notify the SLC of your whereabouts and/or income. However, your loan balance will have continued to accrue interest during the period that you were supposed to be making repayments; settling your arrears balance does not backdate to when the payments were supposed to have been made, so you will owe a little more in accumulated interest than you would have if you had made the payments on time.
You should check the SLC website (see "Repaying Your Student Loan" link in the left hand panel) for details regarding your own specific loan type. Generally they are up front about how they assess and collect repayments, if it doesn't explicitly say they will do something (fines, court action, bailiffs) then you can expect that they won't.
2) Will I need to pay any arrears in full, up front, immediately once
I inform them? AKA is there a chance they will ask me to pay 2.5k
upfront, or can I begin paying a normal amount and they add this to my
student debt as extra debt?
The SLC will ask for the arrears balance in full, up front. However, they understand that this is not always feasible in situations like yours where the amount due may have grown to a substantial figure. They are more interested in working out a payment plan with you than scaring you away, so if you call them after you receive their letter detailing the exact balance due, it should be possible to make arrangements to pay off the arrears portion over a period of several months.
3) I am living in a country where there is no P forms and official
payslips so-to-speak- I'm wondering if they'd accept them as they
probably don't look very official. Does anyone know what would be
considered a 'payslip' and what wouldn't? Ours are literally made on
Excel.
I'm not sure how fussy they are about the format you submit your evidence of income in. They're not looking for P60s or P45s as those are pretty much exclusively a UK thing. The best you can do is submit what you have and see what they say. If they want something more official looking you might ask your employer to write you a letter that summarizes your gross income over a specific period, on letterheaded paper. If they don't accept that then call them and explain your situation, they should be able to work with you. If they document (during the call) what they will accept as valid evidence then you can use that in further correspondence if you run into any trouble.
Some plans assess a "default" monthly repayment amount (it's pretty high - around £250/month) if they can't verify your income for assessment. This may result in a very high arrears balance initially, but as long as you maintain correspondence with them until they are able to reassess the correct amount amount due then you don't have to rush to pay it (in fact, during the dispute process it's probably easier for everyone involved if you don't pay anything in excess of your current monthly repayment amount until you're sure the arrears balance is correct. Otherwise you have to factor in those extra payments when doing your calculations). They will put a note on your account so the collections desk doesn't pester you while you and they are figuring out the correct balance.
I have searched the web high and low and can't seem to find anything online about this.
The SLC repayment website is going to be your best source of information. Unfortunately they focus mostly on what you are supposed to do, not what happens if you don't do what our supposed to do. In most cases however, they will just try to make their assessments when they can with whatever information you eventually do provide. They can recalculate this as many times as is necessary. Their customer service desk has become a lot more cordial and friendly in the last couple of years, so don't be afraid to give them a call.
