Is this a Sedge Warbler?

Hi.

This little beauty flew into my kitchen this morning. I think it had been chased!

It was quite calm and I could pick it up.

Much to his pleasure my grandson managed to stroke it gently.

I took it to the garden and it just stayed on my hand. I had to push it into a bush.

I live in  London SW19.

I've identified it as a Sedge Warbler, but wondered, if it is, then what it was doing in my suburban garden?  

Is it a youngster?

Thanks