There are no railways in Iceland, so you can't use Interrail there. Or do you mean Ireland? That's no problem. You can use a ferry to get there from GB. Some ferries have an Interrail discount (e.g. Stena and Irish Ferries), check the benefits portal.

GB have a Railconnection “ChannelTunnel) and yes you can use the Interrailpass to get to GB (REservation needed for the Eurostar the train that goes through the Channel Tunnel)

as Iceland have no railconnection you can´t use Interrail there.

To get to Iceland you have to take the plane or the Ferry from Denmark :)

€* charges a supplmt for passholders-and you are in summer very lucky to be even able to get one-of 30€=more as WIZZair often charges for a direct PL-GB flite.
there are still 100.000s of rabotniki polskie in GB and in EI=Ireland to help you out in local tips for bezplatne and nizkie ceny.