Coming back from the Netherlands yesterday we were giving a URL when checking in with Stena and told we had to complete an online form for coming back into the UK – no paper option. Lots of stupid questions, no idea whether they were actually checked. We weren’t asked, the car next to us was but they didn’t ask to see the proof.
Can’t advise on Belgium but we no checks coming into the Netherlands from the UK other than standard passport, no checks between the Netherlands and Germany. Stopped at Danish border but only asked about reason for visit, a flash of a Danish passport was enough to get us through there.