Can I use my global pass for this travel? Are there extra charges??
No, you can't. You can buy a "special price" ticket with your pass, but the "special price" is usually much higher than the cheapest discounted normal stand-alone rail ticket (and they come with the same restrictions).
I don't think you can. I don't think Great Britain participates in the Global Pass.
And from 2013 neither will France participate.
I believe that the France dropout is from the smaller Select Passes rather than the Global. I believe that they are remaining in the Global.
As Andre said, a point-to-point ticket booked well in advance (up to 120 days allowed) on eurostar.com will be cheaper than the passholder fare. It can be as low as £39 for a 2nd class ticket. Here's a quote from the Eurostar site: "Passholder fares cannot be booked online. They can only be booked at a Eurostar Terminal or through our contact centre on 08432 186 186 or +44 (0) 1233 617 575. All tickets are sold subject to availability. The 'Passholder' fare is non-refundable but is exchangeable once before the departure of either leg, subject to seat availability at the time of request. The price of a 'Passholder' ticket travelling from the UK to the Continent starts from £58 for a single or from £100 for a return ticket. The price of a 'Passholder' ticket travelling from the Continent to the UK starts from £81 for a single or from £137 for a return ticket."