I'not master but just trying to help you :)
for rist you must to know All of UK IP Address range before.

This just example only:
i make file uk_ip.conf and save on /usr/local/geoip/uk_ip.conf
on file uk_ip.conf put all uk ip address
192.168.1.0/24 uk;
172.168.0.0/24 uk;
and next... uk ip address

One more i want also to add us country ip address
i make file us_ip.conf and save on /usr/local/geoip/us_ip.conf
1.2.3.0/24 us;
2.2.3.0/24 us;
3.2.3.0/24 us;
and next... us ip address

And this file of nginx.conf you can see:
This just example only i say you must use your own skill to configuration your own server

#Nginx Master configuration
user nobody;
error_log logs/error.log;
#Number of worker you need
worker_processes 1;
# How many connections a worker can handle maximum.
events {
worker_connections 50000;
}
http {
include mime.types;
default_type application/octet-stream;
sendfile on;
server_names_hash_max_size 10000;
server_names_hash_bucket_size 1024;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 10;
gzip on;
gzip_min_length 1100;
gzip_buffers 4 32k;
gzip_types text/plain application/x-javascript text/xml text/css;
ignore_invalid_headers on;
client_header_timeout 3m;
client_body_timeout 3m;
send_timeout 3m;

geo $geo {
default default; // for default server
include /usr/local/geoip/uk_ip.conf; // if uk ip address comming found in this database
include /usr/local/geoip/us_ip.conf; // if us ip address comming found in this database
}

// Now you make first default server if that ip address comming no one in your ip address database country
upstream default.webserver {
server default.server1.com weight=4;
server default.server2.com;
}

// you can see i'm put also option weight that configuration will send 80% of the request to default.server1.com
// and another 20% to default.server2.com

// now here put server for access from uk country
upstream uk.webserver {
server server1.uk-server.com;
server server2.uk-server.com;
server server3.uk-server.com;
}

// And nginx load balancing have more option
// Like Option: weight, max_fails, fail_timeout, down, backup more go to http://wiki.nginx.org/HttpUpstreamModule
// Example i will using option max_fails and backup for ip address comming from us

upstream us.webserver {
server server1.us-server.com max_fails=3;
server server2.us-server.com max_fails=3;
server server3.us-server.com backup;
}

// that mean will send 50% of the requests for your-domain.com to server1.us-server.com and other 50%
// to server2.us-server.com if your server1 & server2.us-server.com both fails 3times requests
// will be send requests to server3.us-server.com as backup server.

server {
#access_log on;
server_name your_domain_name.com;
listen 80;

location / {
client_max_body_size 10m;
client_body_buffer_size 128k;
proxy_send_timeout 60;
proxy_read_timeout 60;
proxy_buffer_size 16k;
proxy_buffers 4 32k;
proxy_busy_buffers_size 32k;
proxy_temp_file_write_size 64k;
proxy_connect_timeout 30s;

proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass http://$geo.webserver$request_uri;

}
}

i'm use that configuration for my website but only little different methode :)
hope this can help you to make your server loadbalace :)

Regards,

GhoHan