Thanks all for your input - super helpful!
I use a Mulberry Roxanne satchel, which has loads of pockets and means stuff is easily accessible.
It's great because it's smart and elegant, and big enough that I can fit my laptop in it if I need. It's cross body as well, so means if I'm going to the gym that day, I can wear my gym rucksack AND my handbag.
That plus a use of little pouches for stuff means it's not hard to find stuff in the bag - it's just that there's a lot of it, so I wanted to see how those who don't cart their entire life around with them did it. It seems that having a car would help!
You've been so helpful to chip in, will try to respond to you, given you took the time to help me
Do you need a whole wallet? Why not get a case for your phone that had a section for cards and cash?
Keys, obviously you need those.
Oyster card could go in phone case
Phone is unavoidable
Security pass I'm assuming isn't bulky so should be fine in there
Leave the iPhone chargers at work
The whole point of the charger is for when I'm out and about and not at my desk so I don't have access to a plug, so that I don't have my phone die when I'm out and about. I'm often out and about for a whole day and can't charge if I'm running between meetings or sat on a train with no access to a power point (always request a seat with a charging point, but don't always get one!)
When I'm out and about is when the battery runs down the most, because of email, maps, messaging - I don't really use my phone much when I'm at my laptop and near a charger, it gets used when I'm on the go (when I can't charge!)
What's the tote bag for? Seems unnecessary.
Tote is so that I have a bag to use instead of a plastic bag in shops, and if I need to sling some big stuff in that doesn't fit in my bag (e.g I get something delivered to work that I need to bring home). That takes up next to no space, thankfully
Use small headphones (like the iPhone ones), case should be small then too.
Can you recommend any decent headphones? The Apple earbuds sound is atrocious, and so I've always used proper headphones for decent sound. I should do some research for decent earbud headphones to try out and see if there are any that aren't terrible. This is an area I could definitely slim down on
No need for a whole make up case, just take the essentials (lipstick & powder)
Yep, could definitely do this, and keep a makeup bag in my desk (or chuck it in my bag if I'm likely to head out for the evening after being out and about all day having, and not from my desk)
You don't need to take a whole blister pack of tablets, you can buy tiny pots for this. Cut around the tablet so you still have it in the packet. Then pop this in the pouch for the tissues etc., along with plasters etc.
Good idea, thanks. Though I need to carry up to 8 different kinds of medications with me, so wondering would 8 pots would be a bit bulkier than 8 small strips of pills?
If it's cold enough for gloves put them in your coat pocket
I don't think my coats actually have pockets! I'd better look...maybe a good excuse to buy a new winter coat?!
You need to leave crap like chargers and umbrellas and make up cases in the car and in your desk at work. I only take tampons out if my period is due. Charge your phone while driving and you are unlikely to need extra charging, esp if you turn off internet or put it on power save until you actually need it.
I don't have a car, so can't leave anything there. I live in central London where even when I did have a car, it never got used, as it wasn't practical to drive anywhere.
If I didn't have an umbrella in my bag, I would get absolutely soaked! I walk / tube / bus everywhere, and the weather changes so suddenly that it's very easy to get caught in a downpour. Yesterday was a prime example - the heavens opened and people leaving the tube station without an umbrella got absolutely drenched. If I had a car and could drive everywhere, or was tied to my desk all day, then I wouldn't need to cart all this stuff around with me in the first place
But... I drive to work, and am rarely out in the weather if I don't need to be. That removes an awful lot of stuff from your list!
This is exactly it. Unfortunately I don't have the budget to get an Uber to chauffeur me around everywhere 
Downsize your iPhone charger to a lipstick size one and get a 10cm cable. The best chargers are Anker Powercore Mini and Amazon Basics does a 10cm cable. Weighs nothing and lives in its own drawstring case.
Anker tip is brilliant, thank you! I use an Anker case, will definitely swap to the powercore - awesome tip
Keep two umbrellas in your desk and at home. I only ever carry one if it's obviously going to rain. Best small umbrella is the Totes Supermini.
Thanks for umbrella tip - much appreciated. What weather forecast do you recommend - because I've tried to leave it at home if it's forecast not to rain, and then ended up getting soaked by a surprise downpour. I use a WeatherPro app on my phone - any tips?
You don't need a bottle of water if you work in an office. Have a drink before you leave the house.
In summer TFL issue guidance to always have a bottle of water with you, which I've been massively grateful for when I've been stuck on a heaving and incredibly hot tube and feeling a bit faint or dizzy. Or indeed if I'm on the go all day and don't want to go all day without a drink! Would only have one with me if it's a scorcher, and the underground carriages get uncomfortably hot....
Also my purse is pretty big, and full of crap (old receipts and empty gifts cards etc) so that'd be my first thing...
YES! I so need to do this - so many loyalty cards for places I rarely go to could definitely get ditched.
Can you use a rucksack for your brolly and make up bag etc and then just a small bag for your purse, keys etc
Lots of you have mentioned a rucksack and a smaller bag - is there a benefit to having two bags if you need to cart this stuff around? My satchel handbag is really roomy so it comfortably accommodates everything above. I also forgot to include sunglasses and often a jumper or cardigan
Those of you who don't bother with brollies, what's your secret?! Is it that you drive everywhere? Winter coat with hood makes sense - do you also have spring jackets with hoods as well? Yesterday was pretty warm so definitely not winter coat weather, but biblical rain - I saw people leaving the tube looking like drowned rats, soaked to the skin, because they hadn't anticipated it was going to rain...
What I actually need is a butler to carry everything around! Thanks for your advice (keep it coming..!)