Get 12 months of Apple Music (usually £9.99 per month, so £119.88 for the year), for £79. (£6.58 per month).
Tesco are selling £100 iTunes cards for £80, either in store or online.
Once the iTunes card is redeemed, you're asked if you'd like to purchase a year's subscription to Apple Music for £99 (instead of £119.98), as an alternative to adding the full credit to your phone. The remaining £1 balance is then added to your iTunes account.
So you're getting a £20 discount from Tesco, plus another £20 discount from Apple for the yearly subscription.