While out shopping, find an item that costs less than £5.
Ask your child to work out in their head the cost of 3 such items.
Encourage them to guess first, then let them calculate the answer and talk about how they did it.
If you find something as a special offer, you can use this too.
Something like "2 for £5.70", ask them to work our the cost of just 1 item!
Again, encourage them to explain how they got their answer.
If your child finds some of this tricky, try altering the price to make the numbers more 'friendly'... so the "2 for £3.50" could become "2 for £4.00".