Trees are usually defined to be something very different from cycads and palms. Today if these were growing, they wouldn't be called trees, and their cluster not called forests.
And second, similarly to how 'there's no such thing as a fish', it's been postulated that there's no such thing as a tree. Many different life-forms that we call trees, have no known common ancestor.
I think it's fair to argue that the palm forests noted in TFA filled a similar niche to that of current tree-like forests. All taxonomical groupings are ultimately grounded in practical organisational utility for humans.
As a non-biologist, long term I see tree-ness more as a vocation that plants sometimes take up for a while, more like a hobby than a biological imperative.
Stinging Nettle and Strawberries are related - their common ancestor was something like a tree. It's all pretty confused.
More accurately: the common ancestor of modern 'trees' is often not a tree at all. Like, fruitflies and humans have an ancestor but it's neither fly nor human.
The Earth is 4.5 billion years old. The forest described here is between 358 and 419 million years old, or rather less than one-tenth of Earth's total age.
Australian and Canadian shield formations date back as far as 4.4 billion years. That far exceeds any dry-land life, let alone fully-developed forests.
The formations in which the UK fossils are found were at the time of their formation linked with the ancient Appalachian range, parts of which remain now in north Africa, the eastern US, Spain, France, Britain, Germany, Poland, and Czechia. The Appalachians themselves are older than terrestrial life, which is to say, older than dirt (which is itself comprised of organic matter). There's a river which flows through the Appalachians which is older than the mountains themselves. It is of course named the New River:
Apropos of nothing but flowers evolved at about the same time as Tyrannosaurus Rex, so you can imagine a T-Rex walking through a field of wildflowers at sunrise, its legs wet with flowers' dew.
And, as the response to your comment notes, grass evolved only very late in the age of the dinosaurs.
When you think of the most prevalent varieties of terrestrial plant life on Earth today, particularly agricultural crops (wheat, rice, maize, all grasses), much of it is very recent evolutionary developments, and of that, vastly further developed by artificial selection and hybridisation by humans.
Legumes are also flowering plants, and seem to date back roughly 79--74 mya:
Coincidently, they also discovered 'living fossils' amongst the entertainment acts at Butlins Minehead. Some were said to rival the tree fossils in age.
Trees are usually defined to be something very different from cycads and palms. Today if these were growing, they wouldn't be called trees, and their cluster not called forests.
And second, similarly to how 'there's no such thing as a fish', it's been postulated that there's no such thing as a tree. Many different life-forms that we call trees, have no known common ancestor.
Make of that what you will.