I've heard you can get a cheap train ticket to St Pancras if you are travelling from outside London e.g. Manchester - Paris via Eurostar. How do i get this option when booking a ticket? I don't want to book the whole journey as one journey but rather Manchester - St Pancras, St Pancras to Paris.
worth checking the route (ie is it actually manchester you want it from)
Virgin Trains West Coast offer discounted Advance ticket to London St Pancras CIV (so these would in theory exist from Manchester, potentially in the morning peak) giving both a discount and the CIV protection
But East Midlands Trains do not offer these as 'Advance' discounted tickets from the routes they price (eg Sheffield) - just the usually higher priced savers - which you can in theory buy on the day if you wish - even tho East Mids railway runs trains direct to St P.
You will almost certainly not be able to biy these on line - you will need to go to a ticket office and hope staff know how to issue them (maybe worth having a copy of the Seat61 guidance with you to show the staff)
It's all explained on the link that mirodo has posted however.