If you have an Air Miles account you can get return tickets o DP on Eurostar for 450 miles plus £40. I've not found any travel outlet which offers significant discount on Eurostar tickets, so I've normally booked either direct with Eurostar or the travel has been included in a package deal. I'd also love to know if someone has found a good discounter for Eurostar.
One word of caution if you are thinking of booking for June: the on-site hotels have a very high occupancy rate, and often get fully booked. This might be eased this year with the opening of 3 or 4 new hotels, but I know several people who have not been able too get a room for the dates they wanted because everything (at a sensible price) was fully booked. The same applies to Eurostar bookings if you are booking a DIY package: there is a limit to the number of Apex (cheap) tickets available on each service.
Regards
Rob