One of the main problems with front gardens is watering. Unless you have a front tap or water butts, I would look for drought tolerant plants.
I have a similar sized bay window and I’ve planted it with pheasant’s tail grass (anamanthele lessoiana). It looks fab year round, has no pests, doesn’t need watering, is easy to cut back when it gets too big, but also doesn’t get super tall like miscanthus or pampas grass. I reckon 5 or 6 small pots (9cm) would be enough to give you full coverage around your bay.
I then added some small trees (acers and topiaries) at the front for added height and to frame the bay window. Fatsias and Sambucus nigra would work well too, they don’t mind clay soil and can be pruned to whatever size you want.
Maybe also add some window boxes? I refresh mine with various annuals, perennials and bulbs depending on the season - currently petunias and fuchsias, but also irises and primroses in spring, pelargoniums in the Autumn, etc. This way I have seasonal interest as well as more permanent planting. You do need to be very disciplined with watering and feeding containers, otherwise the plants will just die.
I agree with previous posters that a cottage garden is deceptively hard work. You have to prune back, deadhead, control for pests, feed, water. If you are not a confident gardener, I would recommend you make life easy for yourself by going with a small number of different plants, and focusing on year-round foliage and movement rather than on flowers. Once you become more confident you can progressively add to it.
I am also in the bark mulch camp - and the local cats do not use my garden as a toilet.
One final thing - you can use soil improver like Vitax 6x to break down your clay soil and add organic goodness. A bag goes a long way as it’s very concentrated compared to manure. Good luck!