BAC wrote:

Sessile oak might fit the bill. Wales used to be covered by it.

That's definitely the one to go for. I have seen these growing in poor
soils and harsh environments in Scotland, where they are the
predominant tree in old woodlands where the ground is too poor for
agriculture. The forestry commission say this:


Where and how does the Oak grow?
The Oak likes clay soils or sandy loam soils with plenty of humus.
Sessile Oak is naturally more typical in the north and west,
Pedunculate Oak on the clay soils further south and east. There has
been so much large scale planting of both types that it is now hard to
find these distinctions. The Oak is not always huge and tall like the
great Oaks in the New Forest or on many private estates. In some
conditions it can be stunted and grow into unusual, twisted forms.
Many of the remaining Oaks of the old Sherwood Forest are like this.

Wildlife and the Oak

The Oak is a habitat and community on its own and provides for an
enormous variety of plants, insects, birds, reptiles and mammals - and
that is just above ground! It is late coming into leaf giving an open
canopy that lets a lot of light through. This allows the ground flora
to flourish. A whole range of grasses, flowering plants and mosses are
able to grow and in turn become food for a variety of insects, birds
and other animals.


In upland Oak woods you may find the Killarney fern, Wilson's
pouchwort, the Chequered skipper butterfly, the Blue ground beetle or a
weevil called Procus granulicollis In a lowland mixed broad-leaved
woodland you may even spot a Dormouse.