I keep 1st, 2nd, 5p, and 1p. Those suit my needs pretty well - mainly letters, and some small packets up to about 200g.
With a spreadsheet it's easy to construct a table showing the values of combinations of 1st and 2nd, from which I pick a convenient value at or just below the target. Then I make up any difference with the 1p and 5p stamps. Sometimes when feeling lazy I pick the next higher value and waste a penny or two.
First Second 0 1 2 3 4 5 6
0 0.00 0.32 0.64 0.96 1.28 1.60 1.92 1 0.23 0.55 0.87 1.19 1.51 1.83 2.15 2 0.46 0.78 1.10 1.42 1.74 2.06 2.38 3 0.69 1.01 1.33 1.65 1.97 2.29 2.61 4 0.92 1.24 1.56 1.88 2.20 2.52 2.84 5 1.15 1.47 1.79 2.11 2.43 2.75 3.07 6 1.38 1.70 2.02 2.34 2.66 2.98 3.30(I'm amazed at how well that copied from Excel. One paste operation and absolutely no editing required.)