The oak is Britain's favourite tree, according to NOP. The top 10 were oak, weeping willow, silver birch, apple, beech, chestnut, cherry, pine, maple and ash. Only three of those are on my own list of trees worth planting to celebrate the millennium. Oak, ash and beech are all sturdy British natives, and there are not enough of them about.
You need to take a deep breath before you plant an oak. They are trees for large open spaces. The native oak Quercus robur can easily reach 70ft. Beside the drive leading to Leeds Castle in Kent there is a tree 135ft tall, with a trunk 12ft round. The "Crimea Oak" at Althorp, Northants is 72ft high; at Dunkeld Cathedral in Perth there is one 105ft high. They spread wide, too.
There are good foreign oaks, such as the Algerian oak Quercus canariensis and the chestnut-leaved oak, Q castaneifolia from the Caucasus, but for a millennium planting, only an English one will do. There are various selected forms of Q robur such as 'Concordia', which has bright yellow spring growth, and 'Atropurpurea', with reddish-purple leaves. But it is very slow-growing, and for seemliness and fitness for purpose the common English oak is the one I would choose.