Buy yourself Thermal underwear for the coming winter - no fooling it will repay you handsomely
You could buy yourself a limited travel experience with it, I'm not sure about the people who are saying save, but you could use it for a bus or train ticket into a neighbouring town or area for a day trip just to look around in a different area, if you've spent all the money on the fare take some sandwiches and a flask of tea or coffee and just enjoy!
It's odd, but when we just do the same things every day we don't remember them, but taking a day trip somewhere different can stay in the memory for a long time.
That would be my choice, but of course it's ultimately up to you!
Save it in the bank. Some day you will really NEED that £40 then you will wish you didn't spend it on something useless.
Get something you really want but couldn't normally afford to buy. Like a jacket or tech gadget or something like that. Happy Birthday!! : }
Save it for when you find something you really want that's what I do and then when I find some thing I like (just bought big bang theory season 7) .
Save it for future reference :)
About 12 pints of 'Real Ale' in my local !
You should get two £20 notes or four £10's or eight £5's doh
Save it for when you do know what you want.
I got £40 for my birthday and I don't know what I should do with it