We want to stop people, who can already buy a home, taking advantage of our benefits to later buy the entire home if house prices drop. In fairness, we need to protect our funding partners’ money while helping people - who actually need the help - to become homeowners.
Also, 5% doesn’t sound like much, but it’s still a lot of money. Imagine you start with 5% of a home worth £500,000. Your 5% is worth £25,000 leaving £475,000 left to buy. If your home stays at that value, 5% more would cost you £25,000. That works out at just over £2,000 per month, on top of your rent.