There are many Pollinators in the UK, honey bees, bumblebees and butterflies are probably what most people think about but there are others, moths, hoverflies, solitary bees, mining bees, flies, wasps and beetles also pollinate flowers. Trees, hedgerows and verges are important for pollinators, but our gardens are where we as individuals can make a difference. Pollinators can get lots of pollen and nectar from our garden plants all year. Choose plants with single open flowers for easy access to the pollen and nectar.
Large trees such as Horse Chestnut, Sycamore, Lime and Willow are good for pollinators but they are too large for most of our gardens. Smaller fruit trees and most soft fruit will be visited by many pollinators when in flower. The flowers in my garden which seem to be favourites with pollinators are, Lavandula, Galanthus, Crocus, Deutzia, Nepeta, Agastache, Teucreum, Campanula, Echinops, Phacelia, Ribes, Raspberry, Origanum, Rosemary, and Thyme. There are many other plants and a quick search on the internet will give you lots of ideas. Weeds which have many visitors and perhaps not popular with gardeners are Dandelions, Red and White Clover, Ivy, Blackberry and Borage. Maybe if you have a wild area or a patch of lawn in an out of the way place you could accommodate some wild flowers.
Books
Planting for Honeybees by Sarah Wyndham-Lewis
Plants for Bees by W. D. J. Kirk &F. N. Howes
Websites
https://www.rhs.org.uk/science/conservation-biodiversity/wildlife/plants-for-pollinators
https://www.woodlandtrust.org.uk/blog/2019/03/wildflowers-for-bees/
http://www.sussex.ac.uk/lifesci/goulsonlab/resources/flowers