The one and only time I used Deliveroo I gave a 1 pound tip. A pound or two is a normal service tip for many places in the UK. You would tip your hairdresser or the taxi 1-2 pounds.
I usually give whatever decent change I have laying around, or a fiver if I know them and it has been a while. I never tip via the app. I do tell the driver they'll get a cash tip in the instructions though.
