Is there any place in London that sales GM70 model?

Not according to their website, but why don't you ask them?
got no response with email, can this be a good sign for service quality
More likely the sign of a small company, as is often the case in this industry....
They will get back to you soon.

The GM70s are some of the best amps I've ever heard in my life. Have you noticed how many times they win 'best sound in show' awards?

No, don't own them, but if I had speakers that needed 50W I would.

When I have communicated with them in the past, both George and Kara were very responsive. It is a small company and perhaps they're both out of town or something.
I must agree with Paul, Kara and George do care very much about their customers, Is their a chance you could call them?
As a follow-up - contact James Ridell at sounds real audio if you're nearby. He demos their stuff @RMAF and is a good guy. Hell of a room he puts together.

(No affiliation - never bought a thing from him.)