You have been misinformed.
You can use any 2 days of your global pass for journeys in your home country as well as any other participating country, these are part of the overall allocation and can be taken at any point in the validity. If you wish to travel for more than 2 days at home (Eurostar partially being in GB counts as home country travel) you will need to pay for them separately from Interrail
For example you can book a Eurostar journey and use the pass to travel to London earlier that day.
National rail railcards have no status outside of Britain and are of no benefit with interrail journeys. You can of course use them for any separately purchased journeys.
