Actually, I misunderstood how they work, apparently, each £1 bond has an independent chance of winning, but the average return across any portfolio is tuned to 2.2% (or rather they create a prize fund based on this number).
So you can win £25 from a single £1 bond, and buying more bonds will increase your odds of winning a prize, but not your direct odds of winning a larger prize. Indirectly though, if your win rate is higher, you're more likely to feature a larger prize.