The shard you are assigned by SmartThings just has to do with when your account is created. The only time where you live makes a difference is if you are in a region with different privacy laws.
So all UK accounts will be assigned to a UK shard.
US accounts are assigned to shards simply based on the point in time at which the current SmartThings space on a shard gets full and smartthings (not the individual customer) signs up for more space on a different shard.
So it started out with everybody being on na02-East1, then there came a calendar date when new North American accounts started getting assigned to Na04-east2 etc. (“na” means north America)
So if you signed up for a SmartThings account in 2015, whether you live in New York or Los Angeles, you were assigned to na02.
If you signed up for SmartThings account in late 2017, again whether you live in New York or Los Angeles, you were assigned to NA04.
Most companies, and smartthings appears to be among them, don’t try to assign their customers to Amazon servers based on what city they live in. Amazon does a good enough job for most use cases of delivering service everywhere within one region.
The legality issues involving privacy laws are an entirely separate issue, and do require being on a different shard. 
btw, I have no idea what happens to people who have Samsung accounts instead of SmartThings accounts, or what will happen once everybody’s migrated to the new app.