Poodle is DEFINITELY Tom Chaplin. I am a huge Keane fan and would know his voice anywhere. The stage presence is very Tom, he always taps his right foot when he sings for example and often sings in a slightly bent kneed stance. He also has very long skinny legs (6ft 2 or 3 so he's a big lad). Gary Barlow is 5ft 7/8? So definitely not him.
The clues also add up - so in the first episode the dog collars (x4 of them, as there are four members of Keane), have Queen names on them - well who has performed with Brian May and a series of gigs covering Queen? Tom. A couple of the equations and graphs this week on the board would allude to Perfect Symmetry and Cause and Effect - both of which are Keane albums. Dog collar definitely = Chaplain = Chaplin.
Mika has a phenomenal voice, but the tone is SO Tom Chaplin. I was half convinced during Rocket Man, then completely convinced by his one line in I Want It That Way. He threw me off in What's New Pussycat, but in Symphony? No hiding that voice at all. He's also got a solo album coming this year so it's probably a nice lil bit of promo for him.