A long time ago in a galaxy far, far away, I used to work for a rather large airline who did an awful lot of AVI into Manchester.
Firstly a 25Kg dog sounds like quite a hefty creature, so volume wise you may be looking at around USD3000+. I would suggest you contact an airfreight agent in the states that specialises in AVI as it will be quite difficult to get an IATA approved box with Dims suitable for that size of dog.
Also it might be handy to speak to a specialist in the UK about import documentation etc, the major company (or was) if you are flying into Manchester is a company called Skymaster.
As far as flights go, I'd go with the VS route to Manchester, it will be perfectly fine in H5, ambient temperatures in there, it never gets too cold, I mean we imported fish from SIN in H5!
It should be an absolute doddle, in all my time dealing with AVI I cannot remember a single instance of anything going wrong if it was handled correctly.