I don't know about Dover but I have a recent Plus 4 and have been over many bumps and ramps in the last few weeks fully laden (and I mean FULLY laden) without any scrapes. As previously said the key is to go over at a slight angle and VERY slowly. I've only scraped the cross member once before whilst going over a hotel speed bump but it was my own daft fault as it was quite high and I wasn't going slowly enough.
