What height are you aiming for and are you looking for something that needs regular pruning, such as a fruit tree, or do you want something that just grows?
Also, do you need a tree that grows quickly and will be pruned to keep it small, or a tree that grows slowly to a 'small' mature height? For example, a 'small' variety of amelanchier grows to around 4m, but takes 10-20 years to get there, but a rowan, silver birch, maple etc will reach that height in 3-5 years, so will provide quick-ish screening, then can be kept small by pruning. Fruit trees on rootstocks can be many sizes according to the particular rootstock they're on, so there is a wealth of choices!
I love crab apples for year round interest, especially something compact like a weeping one or 'scarlett' (both 3m mature height), but in your position I'd have a good look at small trees on something like the RHS site, narrow down the ones you like the look of and take note of growth speed, best aspect, hardiness, pruning needs, etc, before choosing. Enjoy! Choosing and planting a tree is great fun!