You may know that there is a major problem re. red diesel (marine diesel) whereby the UK Government proposes to ban its use for all pleasure craft outside UK waters. This means crossing the Channel etc. would be illegal, since only red diesel is available in all UK marinas etc. Conversely it would be illegal for EU yachts to refuel in the UK!
If interested please read the RYA website material. The declaration is being amended and will not prevent you from going abroad except where you fall foul of local (especially Belgian) regulation. The petition to the government won't fix that - just don't go to Belgium!