The best way is to ask. With the ever changing landscape of devices, automatically redirecting is falling out of favor. I would use a cookied little bar at the bottom of the page that asks if they want to view the mobile site. Let them select or close it and have it expire after the session.

One thing to keep in mind, google wants you to redirect to the same mobile page. Like if a user is on site.com/products/nifty-hair-gizmo.html don't redirect them to m.site.com, redirect them to m.site.com/products/nifty-hair-gizmo.html

The reason that automatically redirecting is falling out is because of devices like the new chrome cast and how other phones are set up. I might browse to your site on my phone and be casting it to my 46" lcd and you would never know. But I am going to be irritated if I have to look at your site in a mobile version on my tv.

More than likely it was one 3 things, a DNS issue, a peering issue, or a temp ban.

If you were currently ftp'ing into the site and had too many threads open, usually above 4 or 5 but all depends on the server setting. They can issue a temporary ban on your ip address for the site. Depending on how the server is set up, you can either get an explicit message, which is bad. Or you can just get an error like you, which is good and it means the server is shedding the load.

A DNS issue could be that a name server is down somewhere or having other problems. You generally cannot do anything about this and they are generally fixed quickly because of the amount of sites / information hosted on them is vital.

A peering problem, like a DNS issue is usually spotty. More than likely that is what was happening. A peering issue means you cannot access the "chunk" of internet that the peer directs traffic through. So say you can access 99.9% of everything you want, because it is not going through the peer with the issues.

The best tools you can use to diagnose these problems are TOR, it is a socks proxy that routes your traffic so essentially you will be accessing the site through another isp, who could not be having peering or DNS issues with the hosting isp. Also you can use http://www.whatsmydns.net/  which will let you know what different dns servers around the world are returning. It will let you know if a major DNS server is having an issue. For general checking you can use this as well, http://www.downforeveryoneorjustme.com/

From my understanding, that is not what that tag is for. You can read more about it here, http://googlewebmastercentral.blogspot.com/2013/04/x-default-hreflang-for-international-pages.html

As a suggestion, I would recommend having different content. Just because they both speak english, it does not mean that they speak the same flavor of english. If you can change the content enough to suit the people in the UK I think you will have better overall results.