When you enter your first CTTH tournament, it asks for date of birth and country because the payout model of some tournaments is considered to be gambling by some countries. The country field on the /user/contactinfo.aspx form is disabled - that might be because of the gambling restrictions, but if not, we should be able to enable it. I'll check.
If you haven't provided a date of birth yet, you can use the tournament eligibility form: /golf-tournaments/eligibility.aspx.
Otherwise, contact support and give them your 3-letter country code (if they enter the country name, you won't have a flag). For UK players: GBR = Union Jack, or GBR-ENG, GBR-NIR, GBR-SCT, GBR-WLS.