You should check with your 'carrier' to cross the channel - EuroTunnel or a ferry for what declarations they need.
You don't need to check with any carriers going out of the UK and if you do, you are running the risk of making hassle for yourself where there doesn't need to be any. You are not transporting anything illegal or dangerous. Simply, always ensure the bottle is empty when travelling and have the gun dissassembled.
As mentioned, avoid Luxemberg (not exactly hard to avoid) and keep an eye on this Germany issue.
Otherwise, just enjoy the drive.