She doesn't know that other people don't replace their towels every year
That's insane. Who on earth thinks it's 'normal' to buy new towels every year.
OP, £400 pm on food isn't huge, especially if by this you really mean 'groceries' so it includes things like cleaning products, toiletries and other little bits and pieces you might buy in the supermarket.
You mention Starbucks and that's a biggie. If you normally regularly buy lunch and coffee out of the house, it's easily possible for a couple to spend hundreds of pounds a month on this as a family, so a good area to focus on.
Obviously we've all been forced into this at the moment, so might be a good time to re-evaluate and set a budget and vow to limit this sort of spending to once a week instead of once a day or whatever. Similiarly if you buy food at attractions, usually a lot of money for bad food, so it's not really a hardship to take a picnic, which also saves having to queue up in a normally overcrowded canteen.
Whether or not it is normal to be completely skint before payday depends on your earnings and expenditure. Some people don't have enough money to cover the basics, but it sounds like you don't fall into this category, so it's quite likely you can trim some expenses.
Have a look at the moneysavingexpert money makeover and when you've identified how much spare money you should have after bills, food, travel etc, put some away to save towards annual and irregular expenses like holidays, car repairs, insurance, Christmas, washing machine replacement etc etc. This could be a few hundred pounds a month if you usually have an overseas holiday, run cars etc, so don't underestimate. Then you need to save some money for your new house, unless you plan to just increase your mortgage, but it would be nice to not have to do that if you have money to save.
After all the above has been covered, then's the time to think about setting a budget for personal spends by you and your DH, which should be quite small if you have debts to pay and are looking to move house. You also need buy in from your DH because if he's a spender who carries on as normal, he's just going to be undoing any good work that you put in. Definitely needs to be a team effort.
www.moneysavingexpert.com/family/money-help/