In the 15 years plus I have been travelling the Hoek to Harwich i have never seen anybody stopped or searched when boarding. On getting back to Harwich the thin UK customs post has the capacity to search about 4 motors per voyage. However, border force have stated that animal checks will be away from the usual place they pull people over to search. Seems if you have cites animals you declare them and you are diverted to another site where you may have to sit and wait hours until somebody turns up to check you out.Because the Dutch authorities follow the EU regulations and the French seem to make their own up ad hoc. Its not worth tge risk assuming that its legal to export from the Hook of Holland and legal to import into Harwich thats the way to go. Forget about Dover - Calais.
But from clarifications on Crimbo eve if you have pet/hobbyist non cites reptiles no notifications or health certificates are required for the moment ahead of full documented long-term policy being promised by July.