A new garden! So exciting.
The first things to consider are (1) how much effort do you want to put in it on a continuing basis, and (2) do you have a watering point in the front garden. There’s no point putting in lots of thirsty and hungry plants if you can’t water them, or plants that need a lot of pruning and deadheading if you don’t have time.
Then you need to consider all seasons, and think about including evergreens for the winter, spring flowers (bulbs are good), summer flowers and late flowering plants for the autumn.
Evergreens don’t need to be conifers - Fatsia and aucuba are easy to grow evergreens and a good foil for flowering plants. But they may be too big in a small space. Nandina is small and lovely all year round and does not require pruning.
It’s great that you know some flowers you like. A good starting point may be your nearest B&Q garden centre, they have very fair prices and plants are generally healthy. I would also ask your neighbours if you like their garden - ime gardeners are always happy to talk about their plants ☺️
With roses, there’s lots of choices including varieties that flower continuously over many months.
Peonies are lovely, but the flowering season is very short (2 weeks or so, usually in May or June) and then the foliage is not the most interesting the rest of the year. So ideally they would only be a small accent in the garden - you shouldn’t have them as a focus point. They are also sulky - they don’t like being moved and can take years to start flowering, so you need to be patient.
I would perhaps focus on long flowering and low maintenance flowers e.g. yarrow, daisies, some lillies such as Eucomis, dahlias, salvia (these are just a few examples out of hundreds!). Try and have things with varied height, flower shapes and foliage so you’ve got lots of interesting things to look at. You can decide to have everything in a palette of similar colours (eg white, pale blue and pink) or add splotches of yellow, orange, purple and red.
Also, I would avoid anything that attracts slugs as they can be a massive problem in London.
good luck op