I am unaware of any service that provides through ticketing from the UK to Portugal by rail.
You should check with Eurostar for the Segment to Paris.
http://www.eurostar.com
Alternatively http://www.sncf.co.uk/ may be able to help although they do not have through ticketing to Portugal on their website.
It appears from www.cp.pt (Portuguese Railways) that tickets for the Sud-express can only be bought 60 days before hand at authorised stations. There are none available online.
