Some people like to do an early oil change to get rid of original build swarf ASAP. Any oil that meets the design spec, basically of the correct viscosity for the temperature range and not energy conserving, will work for thousands of miles. The original oil is not a special break in oil. If the plan is to run on the new oil for over 4000 miles, I'd go with synthetic for its resistance to long molecule shearing. Otherwise, non synthetic is fine.
I'm not sure how semi-synth is defined in the UK. If you opt for it for long intervals, pay attention to shifting quality and change the oil before 7500 miles if shifting gets more difficult.
Doing an additional 600 mile change would be a waste of oil. Changing at 100 miles instead of 600 miles is okay.