You should take, or rather have, no less than £80-100 in the bank or as detailed in the latest joining letter.
If you join before about the 12th, first pay to bank will be the last working day of that month. Join after the 12th, pay will go to bank at the end of the following month. Be aware the former may only get a couple of weeks pay, depending when they join, the latter could be paid up to six weeks on their first pay.
This is fully briefed on PRNC and again by your AFCO. You don't need much money, you aren't given the chance to spend much initially.
Initial net pay to bank, after all deductions/outgoings is about £880 to £900 per month for the first six months.