I recommend checking out the christmas bargain thread on here. Loads of offers and voucher codes being shared.
Also vinted. My dd is into manga, ive got her a few box sets for less than a third of the new price. Excellent condition too. I got her some figures, and cosplay. (If you have stuff lying around you could sell to make some funds ready to buy, this is what I do. I use my vinted balance to buy presents/clothes we need).
Less is more, think about what they will really use, need and want. Don't buy for the sake of it. Recent years stockings in our house are mainly edible (treats I would usually buy), bath bombs, stationary etc rather than cheap crap that won't be used/thought about beyond christmas day.
Experience gifts - so the promise to do things throughout the year. There's lots of offers for days out, train Fare can be cheap if booked in advance. I did tickets to Wicked in London for £25 each, London zoo for £4 each, Kew gardens for £1 each, various museums for free. Things like Chessington, legoland etc can be done when the sun does their days out code or on 2 for 1 offers. Daysoutvouchers have quite a few if you travel by train. In my experience my teenager and soon to be teen prefer a day out to more stuff to open!
I got my dd a few gifts from onlinehomestore. They had a lovely rainbow blanket for £7, oodie style hoodies for £9, towelling oodies for £4 (perfect for swimming!).
My biggest bit of advice is set a budget, make a list and only buy from the list. Its all the random purchases here and there that add up!