"The urban landscape accounts for 10.6% of England, 1.9% of Scotland, 3.6% of Northern Ireland and 4.1% of Wales. Put another way, that means almost 93% of the UK is not urban"
And of that 7% that is designated urban, a large proportion is gardens, parks, allotments, sports pitches etc.
Woodland (i.e. Forests) is 12.6% of the UK which is double the size of designated urban areas even if green spaces are excluded.
The majority of the UK is enclosed farmland. So to significantly increase the forestation of the UK, you would need to repurpose farmland.
Unfortunately, they are (at least in Scotland, which is the bit I know about) mostly just tree farms, growing very fast-growing, densely planted pines (Norway Spruce? can't find a reference), frequently in unsuitable areas, and the results were ecologically... poor. Nothing lives in them and they're alien trees anyway. The phrase 'trash forest' was coined.
These days there's a lot more awareness that this is bad, and they're growing more deciduous wood and relying less on monoculture, but it's still a problem.
Not the best picture, but here's a street view shot of one:
https://www.google.co.uk/maps/@57.5275519,-5.186435,3a,75y,2...
(Incidentally, if you look in the opposite direction; that island in the middle of the loch? That's native woodland. The slopes beyond are supposed to be covered with it. But it was all felled centuries ago, then the deer population rose, and now they're making it impossible for native forest to be properly established. You want to promote forest growth in Scotland? Cull the bloody deer down to about 10% of their current population. And then reintroduce wolves and lynx.)