Hi, it is not possible to buy one ticket from UK to Portugal. Most of the time only Eurostar costs about 100-200 EUR € one way. The Interrail Reservation only 30€ on top of the pass. Then about 35€ Paris to Barcelona and from there no direct service to Portugal, but with some changes. Not really possible in one day. 

Varies per country=-there is a list of companies that accept passes-somewhere in the country info. But for the rough trip from GB -via FR-ES to PT about all is included-but for the nitpickers-not each&every train on some pars of this route.

THis 92€ is only the extra you have to pay on top of pass-IF you are even successfull in getting the seats for that price (several are needed). YOu can lower it by leaving most of the his-peed trains aside=makes the trip much longer in time, but you get to see more and the stress is also less.

NOte: that for now, and most certainly in summer/hi-season this is a by now near impossible route to do on a pass-and again you have chosen 2 out 3 of the most difficult countries to use it. This is becse of the needed REServ on top-also the effort you need to do to get them GB has just 1 train going out of this funny island-and this €* is nearly fully booked already. It seems that each&every newbee wants to use the direct TGV FRance-ESpana=same fate (can be avoided as above-use local trains). Connections ES-PT are very scarce and time eating. Read also seat61.com for general advice and overview.