Here are a couple of websites with advice on garden plants, specific to our area: Oxfordshire, with its clay soils, wet climate and mild winters.
Judy Webb is a local botanist, currently advising Oxford City Council on what to plant to help pollinators round the city. She has put together some advice for planting here, based on what she’s observed pollinators foraging on.
Rosi Rollings, a beekeeper, runs Rosybee Nurseries in south Oxfordshire, which specialises in… plants for bees! Rosi has been systematically planting metre-square patches of plants, and meticulously recording what visits them, how often, and how many weeks’ flowering they offer. The site is packed with info of what works in this area, and there is a pdf paper on the Research page with some detailed info. If you look at that, perhaps the key items are the chart on page 19 which assigns a score to various plants, and the chart on page 12 confirming that our area has a June Gap (a shortage of food for the number of foragers), which is why it is useful to have flowers offering food for that period. She will be evaluating more plants this year.