Even though the app is owned by Mastercard, anyone can download and add any credit or debit card to it. Just register put in code QKR10BH1 (generic code, provided by Mastercard themselves so not a personal code), and £10 will appear on app. As long as you spend £11 or more, the £10 will be taken off.
Once you register and put code in, you have 30 days to use it. Must register code by 31 December.
Wagamama is my recommendation of the 3 chains as they NEVER seem to do any promos/offers whereas the others do loads all the time.
I don't see why it wouldn't work to buy gift vouchers, useful for lone diners especially to go 2 or 3 times, if you get a £16 voucher and meals can be under £5 with a 50% off code.