You can get a Square Card reader. Pretty much the same as a Sumup (but a lot smaller). Only problem is that the reader is very, very picky about which phones it will pair with. I got one (when they were on sale for a fiver) and it will only work with 1 phone out of 5 that I tried.
But.....the square App will take manual payments without connecting to the Reader. They do charge a few % more if you use this feature, but at least it's an option if you're stuck with just a Sumup that won't work for some reason. AFAIK you don't need to get their reader for this, just download the App, input your details and you're good to go.