I think that giving a £50 referral bonus is a bad idea for bulb because it makes the motivation external.
For me, an environmentalist, I have the internal
motivation to promote bulb anytime I can because I want to halt the climate crisis.
However, offering the £50 bonus may change the motivation for many people from internal to external. Just like getting paid for a hobby, you lose the passion for it.
In my opinion, bulb would benefit from more referrals and not needing to spend £50 on each new customer if you promote internal motivation over external.
For example, you could send someone who has referred someone else a report showing how much co2 them and their friends together have saved. And maybe post a leaderboard of people who’s network has saved the most co2.