I agree that the online benefit calculators are flawed and can give inaccurate outcomes. I favour turn2us over entitledto, it seems to get more accurate results.
Can you get to a CAB or similar? CABx round my way (and I daresay it's national) subscribe to a fantastic bit of benefit calculation software that appears to be incredibly accurate. However, you need to understand the basics to use it, so it's only used by benefit specialists. CABx have a practice of carrying out a benefit check for every client who rocks up, as this uncovers a lot of unclaimed benefits and errors.
Basically, UC has a range of allowances. All the allowances that apply to you are added together and the total is deducted from your income for the month.
Then the balance (which I think is called excess income in UC) is multiplied by 0.37 and that's how much UC you'll get.
Every time your earnings change, that figure will change again, and the amount you get the following month will go up or down.
But that's not a normal calendar month, every claimant has their own "assessment period", eg from the 5th of the month to the 4th of the following month. If you just have one job, don't do overtime, or get shift/on-call allowances and your pay day isn't close to the start of your "assessment period", your UC should stay pretty much the same. If your income varies, it can get very messy.
Your allowances will include:
Personal allowance £317.82
Work allowance £397
Child allowances £508.75
Housing allowance £0
Your housing allowance will be based on the local housing allowance per month for your area for a 3-bed property. This will be on your last HB award letter or your council's website.
For you, there will be added complications. There will be 2 months in the year where you will get 3 payments from the job that pays you fortnightly. The following month, your UC will fall by 63% of the amount of the 3rd payment. You will need to plan for this. The small variations in the income from the second job shouldn't make huge differences.
Sometimes, the extra earnings are enough to wipe out UC entitlement in the following month, and whether or not this closes the claim entirely seems to be random. Sometimes it does, sometimes it doesn't, and better minds than mine have been unable to get to the bottom of it.
Definitely take your tenancy agreement to the job centre, also proof of child benefit, bank statements, proof of any savings or childcare costs and proof of ID.
If you think you may be entitled to help with your council tax, you will need to apply to the council separately for this. All councils run their own schemes.
Caveat: because I work in one of the last areas to be "rolled out" for UC, my knowledge is limited, but that's what I recall from when I had UC training, and I've just checked the current allowance rates online.
And yes, the way some posters blithely suggest "move somewhere cheaper" shows they have no idea how hard it is to find affordable accommodation in the south-east (and many other areas).
A 3-bed place for £900 a month would be impossible to find round here, and I'm over 50 miles from London. The local housing allowance for 3 bedrooms is £997.08. As LHAs are based on an average of the lowest 30% of market rents, that tells you a lot about how high market rents are.