I don't think it's a bad idea, provided you're comfortable with the volatility/risk of a 100% shares approach.
Could be argued that the state pension is a pretty good bond proxy in any case? It's government backed and it's going to provide a fixed amount of income at some future date.
If you go down the 100% equity route, why go for a 50/50 UK/US split? What's wrong with Europe, Japan, Asia-Pac, emerging markets? I think it makes sense to have some home bias because you're earning and spending pounds rather than dollars, euros or yen, but the UK is less than 10% of the world economy, so having 50% of your portfolio there seems high to me.