One country pass means exactly what it says: can be used in one country only, not valid for travel in any other country or international travel between countries.

If you wish to make that trip with interrail it would be with a global pass.

Reservations are required for Eurostar to/from London and high speed trains to France from Germany, possibly from Belgium as well depending on operator. Thalys and international TGV have high booking fees for interrail.

It may be cheaper to just book the trip with regular tickets from the various operators than using interrail plus booking fees.

Also you haven’t mentioned dates, different countries have various reductions and closures over the Christmas/New year period. For example UK rail is closed on 25th and normally has many closures for the entire period between then and the first week in January.

Thank you.
More clarification:

12/22 Mannheim, Germany - Paris - London

12/27 London - Manchester

12/29 Manchester - London

1/1 London - Brussels - Mannheim, Germany

1. In that case, should I buy Global 4 day pass (677€) ?
   2. Since only Paris - London and London - Belgium require seat reservations, for remaining itineraries we could just hop on the train accordingly, correct? I checked and it is 300€ for seat reservations for both dates altogether.

Costings on not using a pass shows E* returns available for Brussels return about 500 -600 euro for your party size.

UK prices not yet available but should be bookable soon. I assume saver fares will be available for about the same (or less). 

ICE from Man to Brussels not released but looks quite expensive.

So could be a bit less than the pass + reservations but will need prepurchasing. 

Eurostar is non refundable but can be changed for no fee other times (Plus any fare difference)

RES fuer jene €* ist 30€/pp/fahrt. Keine KInd-discount. Also 2x5x30 macht 300-und das ist nur extra. Es gibt jedoch auch quoten-nur ein geringer Zahl aller Sitze in 1 zug werden zu diese sonderpreise an passholders vergeben. Mit 5 koennte dass im hi-season schon knapp werden. Jedoch kann mann jetzt schon auf b-rail nachsehen wie es steht mit Angebot. Es lohnt sich bestimmt auch normalfahrpreise fuer jene tage an zu schauen. Buchen geht jedoch nur wenn mann pass cover nr hat-viele wissen heuer gar nicht mehr das tante DB in Ihre Reisecenter-ein ziemlich grosses gibt es sogar in MA-HBF-direkt ala minute auf Papier verkaufen kann. Es ist jedoch auch sehr wohl moeglich dass es bis dann noch wieder eine Promotion gibt-meist ist es dann 10/15% billiger-aber kein RÚckgabe/Unwechsel usw.

RES: an sich geht dass im ICE ab/nach BRU ohne-aber ratsam ist es nicht-sicher nicht wenn diese Familie Scheu hat ein wenig auseinander sitzen zu müssen. Uber Paris muss mann immer TGV RES-und jene direkt nach MA über Genze kosten dann eine grossmenge mehr. RES im ICE tut mann ganz einfach direkt bei DB-nursitz, zu 4,50.

Da alle Daten nach grosserr Fahrplanwechsel zu rund 12/12 sind-neueres wird es erst höchstens 2 Mon im voraus geben.

Total cost:

Global 4 days =677€

Seat revservations for 2 trips (see pictures above) = 304€

Seat reservations in Germany = 9x2 = 18€ for family

Total = 999€

Question:

1. 12/22: Mannheim - Paris - London. Since I have Global Pass + seat reservation from Paris to London, no extra tickets needed? just hop on?
2. Any seat reservation needed from London to Manchester?
3. Many suggests other alternatives may be cheaper. What are other options? I checked DB Bahn and the schedule is not out yet.

1. As long as you get from Mannheim to Paris on regional trains you are correct.
2. Seat reservations Lon to Man are optional and normally free from station or operator website.
3. Other options are simple - pay for all journeys at standard prices - just wait for schedules on planners and consider various combinations of routings and cost. Alternatively consider low cost flights to UK and then pay for transfers and Lon-Man when saver fares are released. 
4. E* already available for pricing with no pass.

1. You'll need reservations Mannheim-Paris, €16 per person if bought from DB (ticket office or by phone), €15 via Interrail.eu or €13 via SNCF (ticket office or by phone, press #85 for English). If you travel by ICE via Brussels (should also be possible on the way to GB), then reservation from Mannheim to Brussels is just €9 in total (family reservation; price is per journey, not per train).
2. Day trains in GB have optional reservations. You can book them free of charge at any GB ticket office or via GWR.
3. DB schedules from 11 December are as always expected mid-October.

One advantage of the Global Pass is that, because of its flexibility, if you miss a train in the UK or change your plans you can simply get reservations for another one. You can also travel on any train, especially those at peak times, which are not usually available for saver fares.

E* is not so flexible with changes of reservations but that shouldn’t be a problem if you are staying in London the night before.