UK forest are a mix but depending where you are in the country that mix can vary quite a bit as you are likely aware. I tend to use trees and bushes by RMM (search st_rmm) but there are many others some likely better other questionably worse. Building forest don't get too carries away in detail because unless you are walking through them much of the detail will be lost as you are generally traveling to fast to notice. Any of the following trees would not look out of place in a UK forest or hedgerow, but you don't need them all and not all forests are native trees, those planted for timber or biomass are frequently conifer. Search 'tree' and 'trees' and any of the names listed below. Trees by RMM are not named in English but somewhere on either this Forum or the web is a translation matrix kindly donated I believe by another member. Peter
Native: Oak, ash, elm, silver birch, hazel, beech, alder, common hawthorn, cedar, poplar, cherry, chestnut, elder, willow, lime, hornbeam, larch, maple, holly, rowan, sycamore, walnut, yew. aspen.
Conifer: Scots pine, black pine, Norway spruce, Sitka spruce, red cedar, cypress, Douglas fir, western hemlock,