We received many correct solutions to this problem, especially from students at Moorfield Junior School. Thank you all.

Tiffany Lau from Island School showed clearly how she arrived at her solution:

Liam bought 3 lollipops for 84p, so each lollipop costs 28p.

Kenny bought a choco, egg and lollipop for 54p, so choco + egg = 54p - 28p = 26p.

Now a choco + egg + chew = 61p, so a chew costs 61p - 26p = 35p.

Mandy bought an egg, lollipop and chew for 80p, so an egg costs 80p - 35p - 28p which equals 17p.

Since the choco + egg = 26p and an egg = 17p, a choco bar = 9p.

So lollipop = 28p, chew = 35p, egg = 17p, choco = 9p.
Nathan bought one of each, which costs 89p altogether, so he had 11p change.

1 way in which Nathan could spend all his money (£1) is:
2 lollipops, 1 chew and 1 choco bar (56p + 35p + 9p).

Jimmy Ye from Sir John A. Macdonald C.I. School used algebraic notation to arrive at the solution and suggested that there are two other ways Nathan could spend all his money:

Let b represent the choco bar, c the chews, m the mini eggs and l the lollypops.

A set of equations can be written from the known:
b + m + c = 61
b + m + l = 54
3l = 84
m + l + c = 80

we know l = 28 easily

substitute l = 28 into m + l + c = 80 and we get m + c = 52

substitute m + c = 52 into b + m + c = 61 and we get b = 9

substitute l = 28 and b = 9 into b + m + l = 54 and we get m = 17

substitute m = 17 and l = 28 in to m + l + c = 80, and we get c = 35

In summary:
b = 9
m = 17
l = 28
c = 35

When Nathan bought one of each of the 4 products, he had to spend 89 pence, so he got 11 pence change.

There are 3 ways Nathan could spend all his money:
he can buy 3 choco bars, 1 mini egg and 2 lollypops,
or 1 choco bar, 1 chew and 1 lollypops,
or 8 choco bars, and 1 lollypop.