1. United Kingdom is not a member of Eurail, so a Eurail pass isn’t honored there. There is a Britrail pass available for UK, but you need to do a fair amount of travel outside London for it to be worth the price.
2. You can not use the pass to cross the channel, but passholders (either a Eurail pass or a Britrail pass) are eligible for discounts on some ferry lines and on Eurostar.
3. Ferry would be my absolute last choice for a way to cross the channel. 1st choice is Eurostar, the chunnel train. Look for cheap leisure fares and other specials at www.eurostar.com If you can’t find a good fare for your date and you have an adult railpass, then consider a passholder fare ticket on Eurostar. If you have a youth railpass, passholder youth fare and regular youth fare on Eurostar are almost exactly the same. 2nd choice would be to fly. Check Ryan air and use www.whichbudget.com to see which other low fare airlines fly your route. Check our “cheap flights” forum for more links, tips and help. 3rd choice would be ferry — it kills and entire day.
4. Which 15 day Eurail pass are you considering? IMO, a consecutive day global pass is rarely the best deal.

We were planning on using the Interrail continous 15 day card.
We have decided to start at the Eastern side of Europe and work out way over to Lisbon.
Proberly flying to Athens if we can find cheap flights

Sorry, I didn’t realize you were a European resident and thought you were talking about a eurail pass, not interrail. I think UK does participate in the Interrail system, so you can probably use your interrail card there — assuming it covers that zone. I don’t think interrail covers Eurostar and and don’t believe the same discounts and bonuses are available as with eurail. http://www.interrail…