This looks really neat :) But I do wonder how well the Premium Bond (PB) idea will work when not backed by the state (NS&I, who issue PBs in the UK, is backed by the treasury). I would personally worry that my deposit will just be used for prizes in a Ponzi scheme situation.
Another thought is that while it is true that in the UK a large number of the population have PBs, these are usually bought by their parents when they are born. I still have my PB account that was set up by my parents, but I wouldn't consider it a tool for saving. Most people's balance in PBs are a tiny proportion of their total savings. Yotta's 0.2% base rate might change that behaviour though!
Another thought is that while it is true that in the UK a large number of the population have PBs, these are usually bought by their parents when they are born. I still have my PB account that was set up by my parents, but I wouldn't consider it a tool for saving. Most people's balance in PBs are a tiny proportion of their total savings. Yotta's 0.2% base rate might change that behaviour though!