If you're driving into Great Britain from mainland Europe, the single biggest adjustment is that the UK drives on the left, with the steering wheel on the right side of the car. This isn't just a minor habit shift — roundabout direction, overtaking lanes, and instinctive positioning on a motorway all reverse, and it takes real conscious effort for the first day or two, especially for a driver bringing their own left-hand-drive car across on Eurotunnel or a ferry.
Rule 57 of the Highway Code specifically addresses animals in vehicles: pets must be 'suitably restrained so they cannot distract you while you are driving or injure you, or themselves, if you stop quickly,' with a seat belt harness, a pet carrier, a dog cage or a dog guard all listed as acceptable methods. An unrestrained pet loose in the car is a genuine offence under the rule, not just bad practice.
Beyond the restraint rule, standard long-drive practice applies: stop every couple of hours for a dog to stretch and relieve itself, never leave a pet alone in a parked car, and keep water accessible on warmer days, even though the UK's climate is milder than much of continental Europe in summer.