Check out this page ^ and follow the link there to Goods Vehicles: rules on drivers hours and tachographs. It’s a pretty good book tbh, lots of explanation and examples.
Learn these rules, this is “The Book” published by DVSA and is what the wheel tappers use when they do you. It’s a guide book to the law though, not the law itself.
A break taken for tacho rules is a break under WTD and a WTD break is also a tacho break. That’s why the minimum for both is 15 minutes. (A WTD break for factory or office workers has to be 20 minutes minimum, for road transport workers the rules have been modified to tie in with what we already have to do).
Don’t worry about having to learn a new rule book after Brexit, if we change rule books it’ll be to the AETR rule book.
That is the same as the EU rule book, so nothing to worry about.
I keep a diary of my driving time, breaks etc. If I run over time slightly I write an explanatory note about it at the end of my day. If I make a right royal screw-up, I also take a print-out from the tacho and write the same note there as in my diary. 30 minutes over would count as a right royal screw-up. 5 minutes is slightly over.