I have used these tickets - but I live in Germany.
They are a great deal, but they have certain limitations: You are restricted to the connection you've booked, tickets are available 3 months (92 days) in advance and up to 3 (or 2?) days before departure, tickets at these prices are limited - a certain number are available online, another block is sold at the counters and a third block is available from the vending machines.
Your biggest problem is your bike, when you book online. You pay an additional 10 Euros for your bike (to or from Germany, if you don't cross a border it's a bit cheaper) . And you need to make a reservation for the bike. Bike spaces are limited and can be in high demand at peak travel times. Bike reservations can be made up to 1 day before departure (or was it two?), after that, it's up to the conductor whether you can board with your bike. What I don't know is whether you can make the bike reservation and pay for the ticket online and how to do this from outside of the country if you can't. (Maybe there's a hotline in Britain or an office in London?)
I go to the station, make the reservation for the bike (to make sure there's still space for it) and then buy the ticket for myself from the machine or at home online. (Purchasing at the counter usually entails a surcharge. Although sometimes there's a sale and the surcharge is waived.)
As far as I know, the tickets to and from London are all on Eurostar and probably ICE within Germany. You would be subject to the restrictions of the Eurostar. ICE don't take bikes at all.
If I were trying to take a bike from London, I'd use British rail service in Britain to Harwich, ferry from Harwich to Hoek, then the train from HOek to wherever in Germany. Alternatively, you could take the ferry to IJmuiden (from Hull, I think), local train or cycle to Amsterdam and then train to Germany.