britainland:C = a + b*(Y-tY)
C = a + b*(1-t)Y
plus, a = the absolutely minimal amount of consumption to survive
b = a percentage of net income C that is spent
however, this equation is missing a lot even keeping it under the keynesian equation. First, keynes also derives that Savings (S) = I (and we all know this isn't true since the fed distorts the interest rate so we never see this equilibrium, but anyway) so we would have to replace I with (1-b)* (Y-tY) + (tY - G). Also, he is also assuming no taxes without the income tax, which simply is not the case, there are excise and tariffs etc.
Anyway, those are some flaws. I would work through it more but I just got done golfing and am a little tipsy and going to meet my friends.