If it were my son or daughter's birthday I would honesty feel it was better for your child to come than miss the party and I would not expect a present for my child.
It really depends if you have any disposable income at all.
There are tons of really cheap shops round my way and they are falling over themselves to give things at a good price.
I'd try and get some of the two for one or three for two deals. I know things my son loves are things like foamy soup in a Star wars can.
Like this www.boots.com/en/Kids-Stuff-Crazy-foam-bathtime-fun-foaming-soap-225ml_1361286/
B and M Bargains has Bop bags (I think like inflatable punch bags) from £1.99 or £2.99.
I'd save the cost of a card and get your child to make it. Recycle birthday paper too (I always try and save any or neutral Christmas paper).
www.bmstores.co.uk/products/the-simpsons-inflatable-bop-bag-313840
Home Bargains has plush toys for £2.
Disgust
Anger
These are down from £8!!
I fully recognize if you say the birthday child is 12 these suggestions will look weird! 
If you want to go without the present then get a nice card from Clinton or somewhere. They have smashing ones with badges for just a couple of quid.
www.clintonsretail.com/sparkling-fairy-detachable-badge-birthday-card
Anyway, whenever my kids have birthdays the paper gets trashed and I have no idea who bought what!
The only present that may turn heads is a lot of chocolate, which I gave to an adult once when I did not know what to get for a Secret Santa, the adult was delighted but not so sure a child's parent would be (it was about £5 worth of chocolate as the limit was £5 and it was a while ago so I got a lot!).