I came on to wax lyrical about the Whistlefish notelet tins but I see somebody beat me to it! Not on the 4 for £12 offer anymore, although still in yoru budget (2for £10) but keep an eye out, they go on various offers, often.
Jute bags (specifically the small ones which are ideal for a packed lunch. Seasalt does a lovely range, again often on offer.)
Bamboo socks
A mini gardening kit (e.g. herb seeds, bag of compost and a pot) or a mini all in one craft kit (e.g. needle felt a polar bear) or art set if they are at all creative.
Find out which posh brands of cosmetics they like, e.g. Clinique. Ebay is often full of miniatures.
Nice teabags or nice coffee. You have at least two adults you're doing this for so if you buy the type that are individually wrapped you could split the individual sachets of coffee bags/individually wrapped teabags/hotel chocolate individual sachets from a mixed multipack (contrary to popular belief you don't need one of their velvetiser machines, just hot milk, though one of these Milk Whisk Hand Mixer Mini Whisk Electric Whisk Whipper Handheld Beater Coffee Stirrer Useful and Practical : Amazon.co.uk: Home & Kitchen wouldn't go amiss to make frothy drinks!). Taylors of London do individually wrapped coffee bags, and there are all sorts of brands of herbal tea whose teabags inside the box are individually wrapped - Yogi Tea is a favourite of mine.
A thermally insulated lunchbox, or otherwise exciting lunchbox or water bottle - try TK Maxx.
Essential oils (TK Maxx is good for these again)
Scented hand gel (I swear I'm not on commission but I found a lovely lemon scented one by the Body Shop in TK Maxx for £3.99 😂- but also a peppermint candy cane scented one for 89p in Poundstretcher). Bonus if you find the type witha mini carabiner to attach to a bag.
Small hand cream tube suitable for car or handbag.
If they have a dog, a poo bag dispenser, or collapsible water bowl (or bottle/bowl combi) for the car will always be useful.
If they have a horse, they can never have enough spare leadropes.
A Costa gift card (they start from £5)
A microwavable wheat heat pack. You can get them around £2.99 in places like Savers, Wilcos (RIP) and the like.
If they are crafty, look our for the fat quarter fabric bundles in Aldi. Or buy these or find a nice piece of fabric in the bargain bin of a haberdashery and use PVA glue to cover a plain notebook for a handmade gift.
Phone holder for the car
A 3m long USB cable suitable for their phone. Nobody realises how much they improve your life until they have one!
A USB rechargable clip-on-somewhere-handy-light. Ever so useful.
A grabber (I've found one which is £5.99 here Black Plastic Long Reach Grabber, 67 cm - Lightweight, Easy-to-Operate, Ideal for Indoor & Outdoor Use, 1 Piece : Amazon.co.uk: Garden) - not just for litterpickers and disabled people, they're bloody useful when you've dropped something down the back of somewhere!
Pot noodles (well, the posher ones like Itsu) or miso soup sachets.
Those super concentrated squash minis
A bottle of Ribena Winter Spice (Perhaps too big?) - you drink it hot and everybody I know goes mad for it - obviously won't be in the shops until December.
These split between them Mini Clip Top Bottles 150ml Set of 6 Glass Airtight Preserve Bottles With Swing Top Flip Top 150ml Clear Glass Bottles For Oil, Vinegar, Condiments, Water, Juice, Beverages With Tight Stopper : Amazon.co.uk: Home & Kitchen with bonus points if you fill them with nice bits and bobs from your booze cupboard or pantry and use a brown swing tag to label them. If you have things like rosemary or chives growing in your garden you could do flavoured/scented vinegars and oils - the internet will provide ideas!