There's always one outbound and one inbound trip. The test gives travellers from countries like Sweden more days for the O/I trips as the country is very long and travellers from some parts of the country can't reach the border in one travel day. 

Regarding your question, I haven't heard that travellers from the UK are given more than 2 O/I days.

I recently stumbled upon a post from one of my friends who had a 3-day Interrail pass.

Unfortunately, there's a lack of clear information from Interrail regarding outbound-inbound rules.

You can find more information here:  https://eurail.zendesk.com/hc/en-001/articles/11170800337821-Extra-outbound-inbound-journeys-visible-on-the-Rail-Planner-app

I suggest contacting Eurail Support directly at support@eurail.zendesk.com. If you currently hold an Interrail pass, you're typically provided with only two inbound days. However, you can inquire about obtaining an additional outbound or inbound day, and be sure to explain your specific travel situation.