Here's a Budget forecast. There are two issues Brown won't mention today.
1. It's doubtful whether government policies can increase long-run economic growth. This paper says: "There are few, if any, feasible policies available that have a significant effect on long run growth rates."
The Treasury actually seems to largely agree here. It reckons trend output growth from now on will be 2.75% a year, compared to 2.43% between 1986 and 1997 (table A2 of this pdf), but all this extra growth comes from faster population growth.
2. Extra spending on education doesn't greatly improve results; Eric Hanushek gives evidence in this pdf.
Between 1997-98 and 2006-07, UK spending on education rose from £37.2bn to £70.6bn, an increase of over 50% in real terms. (Tables B18 of this pdf and C19 here). How much has the quality of education improved?
Not only will Brown not mention these issues, I doubt if commentators on the Budget will either.
But then, we're not dealing with causal expected utility here, but with symbolic utility. Analysis of the Budget should be done by literary critics, not economists.