Assuming some similarity between the U.K and here in the U.S., it's a little late to be planting anything and expect it toflower or produce this year. I would think about planting bulbs for next spring though. Draw out a diagram of what you'd like your garden to look like and being with some tulips, tulips, daffodils, and hyacinths. It's early yet, since they should be planted untilSeptember or so, but you can look around for varieties you might like. A nice clematis can still be planted as well as some small bushes. One of my favorites is a dwarf burning bush (Euonymus alatus compacta). Other flowering bushes such as Mock Orange are nice for the spring time color and scent.
You haven't mentioned if you like to garden vegetables, but if you do, you could still get in a goodly crop of carrots, radishes and several varieties of lettuce... they do well until first frost. Best of luck! Remember, a day without gardening is a day without a sore back!