I know not everyone prefers Taco Bell and they are not having too many stores in UK. I got one near to where I live and I do visit them ones or twice a week. I found their promotions quite generous compared to other fast food shops. So I'm putting this for those who's visiting them regularly.
1. Ask them for their loyalty card so you can get free food item on every second visit.
2. You can fill their survey to claim 20% off on you next visit. Details will be on their receipt.
3. You can ask for vouchers instore. You can see them on bottom part of picture here.
Most importantly you can stack voucher and loyalty card offers together. So you can get discount on food bill and free food item at the same time. Or you can enter online survey and get 20% off when you not getting free food item. Also you can stemp every time and claim it later on whenever you like.
I know it's bit complicated but please feel free to ask before voting it cold.