I think it’s a very basic level of love to buy each other gifts; and it doesn’t have to be grand or majestic.
They could even set a budget, say £50 and do a stocking each with little bits the other person wants/needs (cute socks, lip balm, DVD, pop socket, mug, gift card etc) and one £30-40 gift would be so much fun. I love waking up Christmas morning lighting the fire, listening to Christmas music and sipping Bucks Fizz with my partner while we exchange gifts over an hour so so (because we drag it out, discuss gifts, take photos - just for us- fuss the cat) it’s a weirdly intimate & romantic thing for us!
For anyone else, I’d say if you can’t afford it then it absolutely isn’t required but she can spend £36 on a candle and he can get her a Tiffany necklace for her birthday but nothing at Christmas? Nah she is just selfish and will buy what she wants for herself and therefore won’t buy for him.