A great way to earn additional Avios in the UK is via Tesco. Basically, you can convert your Tesco vouchers into Avios (either BA Avios or the scheme formerly known as Airmlies, now called Avios) at a rate of 600 Avios for every £2.50 worth of vouchers.
However, its not only shopping a Tesco that lets you do this. Another great way is via the Tesco Mastercard. We’d recommend putting all major spend onto your BA Amex card, but there are still a number of places that don’t accept Amex, so you should use the Tesco Mastercard as your backup. Tesco give you 1 Clubcard point for every £4 of spend, and this works out at roughly 0.6 Avios per £ spent after you convert. This is obviously not as good as 1.5 Avios per £ spend with the BA Premium Plus card (or the 1 Avios per £ spend with the standard BA Amex card) which is why we recommend it as a backup.
You can apply online for the card here: