Eurostar offers fares from London to Metz from £99 return. London - Paris can be booked up to 4 months in advance and Paris - Metz up to 3 months in advance and these through tickets can therefore also only be booked up to 3 months in advance. Alternatively, you could buy a £69 return from London to Paris and onward tickets from
www.tgv-europe.com though this is likely to be more expensive.
If you buy a ticket from Wigan rather than London from Eurostar, you will be offered a ticket from Wigan to Lndon Estar (CIV) on a connecting train but you cannot use Railcards or choose an earlier train (which may have cheaper Advance fares). You will also have to wait until Wigan - London bookings are available before you can buy such a ticket (in which time London to Metz prices may go up). It is therefore advisable to use Eurostar Domestic Sales (01233 617913) to buy a ticket to Lndon Estar (CIV). These special tickets for Eurostar connections are only available through Eurostar, give extra protection in case of delays and are usually cheaper. Alternatively you can buy a ticket to London Intl (CIV) from any railway station - the protection is the same but the price may be different (usually higher).
There is a local train (TER) from Metz Ville to Gandrange-Amneville. The whole journey is likely to take around 9 hours.
Singles with Eurostar cost just over half the price of the return so a return for London to Paris / Metz would be cheaper than two singles. However, Advance (or equivalent) fares for Wigan to London and Paris to Metz are only sold as singles.