Answer: £10

Derivation:

1. By reverse calculation:

The final amount you had (which you spent on Thursday) = £2.50
This amount was left after you spent HALF of you had on sweets. As you spent HALF of what you had, what remained (£2.50) should also be HALF of what yo you had. Thus, before spending on sweets, you must have had £5.
Now, this amount was what was left after spending one-third of what you had, on CD. Since you spent one-third, what remained (£5) must have been two-third of what you had before spending on CD. Thus, the amount you had before getting the CD = £7.50.
Finally, this amount was what was left after spending a quarter of what you started with. Since £7.50 was the balance after spending one quarter, it means £7.50 was three-quarters of what you started with. Hence the initial amount (the pocket money you got) must be £10.

Put differently:
Let the starting amount be X.   (1)
You spent one quarter thereof on cloths. Balance = 3X/4  (2)
Then you spent one-third of this on CD. Balance: 3X/4 x 2/3 = X/2    (3)
Thereafter, you spent half of this amount on sweets. Balance = X/2 x 1/2 = X/4.   (4)
The amount remained was £2.50. Since this is one-fourth of what you originally had as per (4) above,  X = 2.5 x 4 = 10.