Travelling by train from London(England) to Cologne(Germany)

A rail pass probably won't be worth it for the international journey. You can get a one-way fare from London to Cologne for only 59 euros if you book at www.bahn.com 92 days in advance. These fares aren't available for journeys using Thalys, but only for ones using German ICE trains between Brussels and Cologne, which are also high-speed. If you miss the discount fares from bahn.com, you may be able to get a slightly more expensive, but still discounted, ticket from the Eurostar website.

melmcclan, DO NOT buy a rail pass to get from London to Germany. For travel within Germany, only consider a rail pass after listing the trips you want to make and adding up the cost. It will almost certainly be cheaper to buy advance purchase tickets for longer trips, and local buy-on-the-day tickets or local passes for shorter trips.
The man to consult is the "man in seat 61". For London to Cologne, see here: http://seat61.com/Germany.htm#London%20to%20Cologne%20by%20train
For times and prices within Germany, use the DB (German Railways) website: https://www.bahn.de/p_en/view/index.shtml

Also try this site:>https://loco2.com

I don't believe EuroStar accepts rail passes, but I haven't checked recently. For the few trips you'll be making, using P-P tickets is probably going to be the easiest and most cost effective method. You may find this website helpful, especially the part "Should you buy a Eurail pass or regular point-to-point tickets?" It also has good information on the EuroStar.

http://www.seat61.com/Railpass-and-Eurail-pass-guide.htm#railpass-or-point-to-point-tickets

Also, I didn't notice when I first read your post, but travelling from London to Cologne via Paris is very indirect. The best route is Eurostar to Brussels and then either Thalys or ICE to Cologne. If you went via Paris, you would be passing through Brussels anyway.

I don't think the Eurostar to Paris passes through Brussels. Doesn't it stop in Lille and turn towards Paris? Other comments here are entirely correct and helpful for the OP.

For Tim's benefit, The Eurostar trains from London go to either Brussels or Paris, but not both, it is a "Y" shaped route. The point where the two routes diverge is Lille. But, in the current timetable, only trains to Brussels stop in Lille, the Paris trains are non-stop from the Channel tunnel to Paris, there are separate TGV trains from Lille to Paris.

I too missed the Paris reference. Do not go via Paris, the direct route is via Brussels, then a Thalys or ICE High Speed train direct to Cologne.
I too recommend https://loco2.com/ for buying tickets. It is a UK site, prices are in £, and it gives the same prices as the railway company sites, but it offers through tickets, avoiding having to book the two legs on different sites.

To confirm speculation above, Eurostar trains from London do not accept rail passes.

I would look at just the German Rail Pass. It covers whatever your plans may be in Germany as well as covers ICE trains (not Thalys trains) departing Brussels Midi/Zuid station around 9:25, 14:25, and 18:25.

I meant that if you travel from London to Cologne via Paris the route would end up being London - Paris - Brussels - Cologne, not that Brussels is between London and Paris.