You’d be surprised how much fuzzy socks cost now. At least £8+. More like a tenner even in cheap shops. And then mug will have been £5-£10 too. So honestly sounds like they did spend around £20.
I volunteered this year to be part of a homeless/refuge gifting initiative, and was assigned a mum in refuge. The aim is to put a gift bag together of things they may like as they’re very unlikely to receive anything for Christmas (DV situations etc). I thought it’d be about a £20-£30 spend but have easily spent about £50. The suggestions for gifts included things like fuzzy socks, candle, chocolate. I ended up buying things like basic toiletries, tweezers, nail clippers, mascara, lip balm, hair towel, hot water bottle, a fun Christmas cup, a nice candle, sachets of hot chocolate and coffee (the kind with milk already in, so they just need water), some snacks bars and a little selection box. I had in mind a friend who fled a DV situation with her baby with nothing more than what they were wearing, so it was a lot of ‘essentials’ plus some ‘fun’ bits. I never did buy fuzzy socks because I couldn’t find any under a tenner! Which is honestly ridiculous, because you can get a 5 pack of normal plain socks for less than that.
Anyway, my point is that everything costs more these days, especially around Christmas, so try not to take it too much to heart because I bet your gifts cost more than you thought. I bet the small Celebrations were to bring the final total up to the limit.
As for your colleague not thinking much of you, they might like you a great deal but not know too much about what you’re into so went with some ‘safe’ choices. It can be so hard to buy something lovely for a colleague if you don’t really know them that well personally (beyond the surface stuff most people tend to talk about at work).