Acer would work well Not if it's an exposed site, they don't like draughts.
Apples are not generally self fertile (although there may be trees in neighbouring gardens which can act as pollinator). They are listed with their time of flowering so you can be a partner which is in flowr at the same time. However, you can get "family trees" - several trees on the same rootstock - so they will pollinate each other.
Crab apples are nice, with flowers, followed by attractive fruit.
And am I correct to think most trees need to planted in the autumn/early winter, or could I get one now (or when/if the lockdown ends)? If it's grown in a pot, you could plant it now, although you'd have to water it heavily over the summer. If you order on-line, it's likely to be bare-root, so for planting over winter.
However deciduous trees aren’t pretty In winter They can be - Betula "jaquemontii" (one of the silver birches) has really white ttrunk and branches which look stunning in winter, Prunus serrula has shiny mahogany red bark which makes you want to stroke it, snake-bark maples have green and white sriped trunks, to name just a few. Crab apples like John Downie tend to hold their fruit most of the winter.
Evergreens can feel fairly boring because they don't do much - apart from possibly a short period of flowering they are just green. Whereas deciduous trees change through the year, from fresh spring foliage, yellowy green or sometimes pink, through rich green summer colours, then the autumn colours of gold, orange to red, to the winter look of a fine tracery of branches against the sky. And you only have to clear up leaves once - evergreen drop leaves all the year round.