You say your car 'isn't worth much' but it must be worth a few £k and be expensive to fuel, tax, insure and maintain, being a big Audi.
If you sold it and bought a small sensible car like a Skoda Fabia or a Honda Civic, you will release some capital and save on running costs.
Have looked looked at minimising all your costs? (gas/electric/insurance etc) - if you haven't done this before, you can save loads. Have a look on the budgeting section of moneysavingexpert for a step by step 'take a day to save 1000s' guide. Even if you only saved £20-30 a week, it would increase your budget for food and other costs significantly.
If you saved £20 per week, you could probably afford a holiday abroad every couple of years or so, so hardly 'your children never having another foreign holiday'. However, you would also have to look differently at what sort of holiday it would be - I imagine that you have previously been long haul, or skiing, or luxury resorts. This wouldn't be possible, but if you booked independently, you would be able to get an apartment and budget flights, and still have a relaxing week in the sun.
I think your main problem is, and I mean this in the nicest possible way, is that you aren't used to thinking like someone who has to live on an average income and needs to budget, hence your comments of a £30k salary job being 'crap' (it is actually well above the median salary and also above the mean average salary, which is skewed by a minority of very high earners). I bet your car isn't what most people would call a banger either.