Could do with some clearer pictures and more info (flowers/fruit/height/place).
Superficially it could be a European beech, Fagus sylvatica.
Look on the ground for last year's fruit (if beech, it would be small brown husks)
Thinking out loud...
I wondered about Cornus (although most have opposite leaves, some species have alternate ones, like yours). But the leaves in yours do not have sufficiently prominent main veins and they appear not to be toothed.
...Ditto Carpinus, Ostrya (though leaves in the background look like Carpinus/Ostrya or even Ulmus)
Some maples have green trunks as young trees, but they always have opposite leaves.
There is a superficial resemblance to some roses, but too many things wrong.
Celtis: leaves not pointed enough.
Salix: leaves not right, even for a sallow.
Nyssa sylvatica: leaves about right, but twigs grey-brown (not yellowy green like yours).
..Ditto Rhamnus, Elaeagnus, Cydonia
Ligustrum, Euonymus, Vibunum, Prunus - no
Nothofagus - various things wrong
Cestrum - leaves too narrow, +
I don't get any Betula (birch) to fit, either.
Alnus viridis is similar in some ways, but still not right.
There are quite a few species of Cotoneaster that key-out as fitting (first link below), and I haven't checked them all individually, but it doesn't look right for Cotoneaster in general.
...So if it is a British native or commonly cultivated tree/shrub, I don't know (or I am going to kick myself).
Any chance you could find out from the landowner?
Are there lots of them in the area, or is this the only one(/few)?
Info in fruit/flowers would be really helpful... (maybe on another visit?)