I think it's Cornus kousa, also known as Korean dogwood. It's hardy and grows to about 7 metres tall and wide, so needs lots of space.

It's a lovely tree and as well as the white blossoms (not proper flowers, but coloured bracts) it has berries that the birds like. In my experience it doesn't grow very fast but that may have been the soil in my last garden. It's easy to cut it back when you need to.

Think it takes about 20 years to reach 7 metres.

Thank you. Now to decide whether there is space for one in my garden.