I also wanted a 6xx series but ended up buying a 300s in a local dealer instead as these are far more commonly available in the UK. Some 6xx series do come up on the gun selling sites in the UK if you are prepared to wait. I've seen a few since I bought my 300s. There are far more available in Germany but like you I didn't want the potential hassle.
If you want to take a chance however, an alternative to egun is the following:
As you can see if you scroll down the page there are a number of FWB 6xx series rifles for sale. Although it still means buying from Germany, it's a classified ads site so you don't have to bid and instead just contact the seller to agree a sale (probably with the help of Google Translate!). Some will also post abroad including this nice example:
Good luck and I hope you find something.