It really depends on what you’re looking for, there are products/brands you can only find on some sites. I personally buy from bike-components more often, but I rate bike24, actionsports, bike-discount, rose, r2-bike just as high.
There are no hidden costs, owing to the fact that Germany and UK are both in the EU. Do note that some of the sites charge German VAT (19%), while some might charge the UK VAT (20%) – this will be shown at checkout. I found this to be true with Bike24, although I’m from Romania.
Given that the Touring is essentially an 800 with a different firmware, the mapping software should be the same. The preloaded maps differ between Touring and 810, as far as I understand.