To stop users being prompted for their region and timezone when they first log into Outlook Web Access you can run the following powershell command to set the value for all mailboxes.

get-mailbox -resultsize unlimited | Set-MailboxRegionalConfiguration -Language 2057 -TimeZone "GMT Standard Time"

A list of country codes are here: https://msdn.microsoft.com/en-us/library/system.globalization.cultureinfo(VS.71).aspx

The to check the correct value has been applied use the powershell command:

Get-MailboxRegionalConfiguration -id [email protected]

Only applies to current mailboxes not future ones.