My 2p - the Eurotunnel Bike Service (le Shuttle) is brilliant - £16 each way per person, incl bike. As stated up-thread - you meet the driver at the Holiday Inn Express and and he drops you off by the Designer Outlet place near Cite d'Europe (and vice versa)
We used it last year, both ways, for 9 of us - they dropped us at Folkestone West Station on the return journey - this year we were 13 for just the outward journey and got the driver to meet us at Folkestone West as we arrived by train from London. The official transport figure is 8 per trip - if you ask they're very flexible - this year bikes were packed carefully in a van, with blankets over them, and we rode in a 16 seat minibus. Excellent.
... for the Eurostar (i.e. passenger train) we checked bikes in at Gare du Nord - just wheeled them in, no bag - last Tuesday - and picked them up at St Pancras (we were a group of 13 and got a special rate of £20 per bike + the group rate passenger fare) The trains take a max of 8 bikes so we had 6 on one train, 7 on the next, which we also travelled on. Absolutely no problems at all.
NOTE : The 120cm luggage/bagged bike rule is now about 85cm for carry-on ... that means that even a Dahon folder only just scrapes into the size.
... and Calais/Frethun has very few trains stopping there ... and fares tend to be very, very high (London - Calais is much more than London - Paris)
If you've been by car through the tunnel you'll realise that riding to the terminal, loading the bike in the train etc is not realistic - hence the off-site pick-up.
The tunnel is the ONLY way!
Rob