Default timezone for new user is PST not GMT

We are a charity based in the UK and when I set up a new user their timezone seems to get set as PST rather than GMT. I selected the licence location as the UK and I don't see anywhere else I can see a location/timezone, so I have the following questions:

1. How can I ensure future new users are in the GMT timezone

2. How can I change current users in the wrong timezone?

For example, when I try and register a meeting and invite someone who is physically in the same timezone as me, the following message appears in Outlook "*user name* is in a different time zone. Use the Scheduling Assistant to find time across time zones."

Thanks