I’ve been over a couple of times and haven’t had to do anything. Just drove over the bridge and never heard anything. They use ANPR, so it is a lot easier than other crossings where you have to register first. We live near the Tamar and to cross from Cornwall to Devon you have to pay a toll (either on the bridge or ferry). The hoops one has to jump through to get free crossings is not worth the trouble if you only cross a few times a year (it only costs £2 for a car).
Current lease: 2025 Omoda 5 Estate 1.6 TGDI [147] Noble 5dr 7DCT
Last lease: 2022 MINI Hatch 2.0 Cooper S Classic Premium 5DR Auto
First lease: 2018 SEAT Leon Sport Tourer 1.8 TSI FR Technology DSG