I think £1500 should be an ok amount to live on too when you are mortgage free.
We try and run off Dave Ramsay percentages, in your case it'd be something like this:
Savings 10% - £150pm (Aim for £1000 emergency fund, then a 6 month emergency fund) - £1800pa
Food 15% - £225pm - £2700pa
Utilities 10% - £150pm (Gas/Electric/Broadband/Sim only deal) - £1800pa
Housing Costs 10% - £150pm (Council Tax only) - £1800pa
Transport 20% - £300pm (small car payment & fuel) - £3600pa
Insurances 10% - £150pm (Life, Car & Home) - £1800pa
Recreation 10% - £150pm (Netflix, socialising) - £1800pa
Personal Spending 7.5% - £112.50pm (Clothing/footwear) - £1350pa
Miscellaneous 7.5% - £112.50pm - Whatever you want - £1350pa
100% - £1500pm - £18,000pa take home pay
So a rough idea breaking into categories and percentages. The biggest expense will probably be transport. Maybe you won't have a car payment or a car at all and decide to use public transport. It's a starting point and you can move funds about the categories.
Another poster said you may get half his pension, so maybe your income will be more than £1500 and NMW is to go up next year too.
Food will be tricky, you might need to up it. If you've had a big lifestyle, maybe you've never had to budget for food and that may be hard and a learning curve for you. Maybe get some budget cookbooks, mealplan, switch to aldi and lidl etc.
In an unrelated note, if you have been together for a long time, have you discussed your marriage, its a big change kids getting ready to move on independently.
Have you tried marriage counselling or discussed why you are unhappy, is he also unhappy?
Maybe you both are a bit lost individually, maybe you have both lost your identity and that's hard.
Raising a family is very hard, you've both done the hard part, this should be time for you to look forward to time together.
Can you plan some trips together.
Watch some videos about marriage - jimmyonrelationships on youtube is quite good.
Do you have regular frank and open conversations about your marriage? Does he discuss it or bring it up?
If neither of you are discussing it, there's no infidelity or abuse and you are secretly planning to leave, it seems a bit of a waste without trying to give your relationship the time it possibly needs.
Maybe you feel neglected, maybe he feels neglected, maybe the marriage feels neglected. Sometimes these things are fixable. x