Good underwear. Go and choose a couple of sets from house of fraser/John Lewis type place where they will measure your bra size as it might be different if it's ages since you bought stuff. Treat yourself to good Walford/falke tights. £100
Then jeans. Nudie jeans are inexpensive expensive jeans at around £100. Paige are lovely too and will last and look classy and you won't regret spending on them. So, jeans def allow £100 but up to £200
Dress. What would you like the dress to do? If it's feminine daywear then would this be the sort of thing? Say£100
Now it's a question of coat/boots/blazer - which ever is most important in your wardrobe. You might be able to get two depending on how much you spent on the jeans. £200-£300.
Whatever you decide on, don't try to do too much. Don't waste this opportunity on accessories as you can probably make do with the ones you've got, or ask for them as gifts or even treat yourself to them in the future. Justifying £200 jeans is going to be harder than a scarf from accessorize.
Also, take the opportunity to spend on underwear - it should make the clothes you've already got look perkier so make you more enthusiastic about revamping yourself from your own stuff.
Good luck with it all. I'd love to hear how you spend it! Please come back and let us know. What a brilliant way to blow away the pnd, and what a lovely family you have!