I try and tell as many people as possible about DICE because I just really like the product.
That said, a referral programme might mean that there's that last little bit of incentive to actually send a link to someone and make sure they follow up on it too. I think it's almost more about the new customer being incentivised to give it a try.
A couple of quid would be enough of an incentive for me but I don't know how it would differ for people generally. I know that with some other apps (e.g. Wriggle, Deliveroo) a referral of £2 or so has felt like it's really not worth it if you have to spend £20 to get it.