Why not floo instead of thestral to London?

[Hitomi]

No: HPFGUIDX 91671

Sue wrote: 
> I have always been troubled by this as well.  When Harry put the 
> mirror away, he vowed never to use it and, evidentally, forgot all 
> about it.  Why, when he broke it at the end did he throw it back 
in 
> his trunk instead of in the trash?  Where is James's mirror, we 
know 
> he had one.  I have a feeling the mirror (and perhaps all mirrors 
in 
> the series) will reappear and Harry will be very glad he has it.  
It 
> is, of course, in several pieces now.  The better to share with 
> others he needs to keep in touch with.

Hitomi:
I always assumed Sirius had both mirrors, and gave James's old one 
to Harry (according to Sirius' note).  But the talking mirrors have 
always intrigued me, like the one in the Leaky Cauldron in the room 
Harry stays, or the one at The Burrow.  And they all seem to like to 
comment on Harry's hair, and overall general scruffiness.  Are they 
similar to portraits, or do they stand alone; and unlike the 
portraits, what give thems a personality, or is it just some sort of 
Charm?  

But you're right, such a communication device would be very handy 
for the trio.

~ Hitomi, who really hopes JKR put the mirror in as some kind of 
plot device, otherwise it will be one thing about the series that 
will always annoy her