Pearlsasinger
Up in the clouds
I have kept 2 horses on half an acre perfectly well, it is quite adequate. Where I live it is completely normal for horses to live on much smaller areas than the BHS suggests as preferable, but then, we feed hay year round and do not rely on the acreage to sustain the horses.
Obviously it will basically turn into a dry lot and forage will need to be offered year round. That said, if the ground is clay or something that floods then I would say no, because it will be a bog in the rainy season, on sand, or well draining soil then it will do.
It's obvious from this post that you haven't been in UK for sometime, Enfys. This has been such a wet winter that it would be difficult to find any land dry enough to keep 2 horses on such a small acreage, here.