Iris pseudacorus I believe.

As I couldn't get up close to this plant I can't be sure that I have identified it correctly. Iris pseudacorus.

 

Certainly looks like it, the only thing that has me uncertain is the date - at least where I am, it won't be flowering for another 6-7 weeks yet. When were the photos taken?

 

Hello Michael, I live in S. Italy which is definitely much warmer than the UK.

 

I did wonder if that might be why, but it is so very much earlier than here!

 

Trees that bloom in March here may bloom as early as December in California some years. A yellow flag I planted by mistake (misidentified by nursery) in my friend's cool garden is up pretty high already (although not even in bud yet). So seeing one in bloom now in California wouldn't surprise me at all (here vs southern California would be pretty close to Britain vs lowland Italy).