Personally, I like this Idea because as anyone who has ever held premium bonds know they can be a bit of a pain to purchase, manage and sell.
But (and this is where I have to bring that horrible legal bit out), from a legal standpoint lotteries are super regulated and Camelot who run the national lottery have been known to be agressive towards competitors - http://www.bbc.co.uk/news/business-19343046.
Nonetheless it’s a good and different Idea, if the legal bits could be worked out it would be an interesting concept and probably quite popular.
Maybe a less legally sensitive idea would be a competition each month where there were prizes (in lieu of interest), where your balance didn’t affect the number of entries (making it a competition not a raffle/lottery) which would perhaps be legally easier.
Lloyds did this a while back http://www.telegraph.co.uk/finance/personalfinance/bank-accounts/10416942/Lloyds-to-refund-1000-card-purchases-a-week.html which seemed to be ok as it was a competition rather than a raffle so maybe it’s possible.