The cost of one model aeroplane is £3.40.
When Tafsir buys two aeroplanes, the total cost without any discount can be calculated as follows:
\[ \text{Total cost} = 2 \times £3.40 = £6.80 \]
Tafsir saves 20% on this total price. To find the amount he saves, we calculate 20% of £6.80:
\[ \text{Savings} = 0.20 \times £6.80 = £1.36 \]
Therefore, Tafsir saves £1.36 altogether.