Definitely easiest to just jump in the car and drive over via Eurotunnel. Other than the fuel the actual costs of the train through are very reasonable. I did it when driving to Brussels last spring to pick a dog up. I left Brussels at about 7am and was back in the Scottish Borders by 6.30pm same day. Not sure about the distances I valves for you but an overnight stop off could be factored in??? Providing all the legalities are adhered to with regards to inoculations, paperwork and the dog being chipped, the drive back with the dog requires a stop of about 10mins at the designated office just prior to boardingthe train in your vehicle to have the chip scanned and pet passport stamped. A friend last year looked into a courier bringing a dog back for him and I am sure he was quoted about a grand!!!
Just looked at your location so you have a fair distance to travel as well. I stopped over night before going on train in Felixstowe and got an early train through. By that reckoning you could doit all easily in 3 days if your destination in Germany is a long ways off... Fair bit of driving though...