If I'm in the UK, can I only publish an iOS app to the UK app store?

When you configure your app settings, by default your app is sold world-wide. You can choose to restrict it by country.

Not all countries have access to iTunes. From wikipedia:

A user must also pay with an iTunes gift card or a credit card with a billing address in Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Republic of Ireland, Italy, Japan, Kenya, Luxembourg, Mexico, Netherlands, New Zealand, Norway, Portugal, Puerto Rico, Spain, Sweden, South Africa, Switzerland, United Kingdom, or the United States. Apple also offers other payment methods (like Paypal), which differ from country to country. Residents in other countries can only buy a gift card from a merchant or download free podcasts and previews.