Oh dear, lots of misinformation on this thread.
If you lose your job you will be able to claim new-style Jobseekers Allowance (nsJSA), this is assuming you worked enough to have a full NI record for April 2019 to April 2021. nsJSA is only £77/week though.
You haven't said how many children you have and how old they are. But you will be entitled to Universal Credit (in addition to nsJSA) as it includes the child element.
You won't be entitled to help with your mortgage costs although if you are still unemployed and on UC after 9 months you could get an SMI loan - note it's a loan.
You will also be able to claim Council Tax Reduction (sometimes called Council Tax Support, depends on the council).
You may be entitled to the cost of living payment, warm home discount, and other support available to people on UC.
Your child maintenance income and your savings (anything under £6k) will be ignored. If you have space to take in a lodger, income from the lodger will also be ignored.
Do you intend to look for another job or is there a reason you might not be able to get one? If you're likely to be unemployed in the longer term and unable to pay your mortgage you will need to seek more advice.
You can contact Citizens Advice as PPs have said.