The People’s Trust for Endangered Species (PTES) and the British Hedgehog Preservation Society (BHPS) produced a report called the State of Britain's Hedgehogs in 2018. In it they chart a decline of half the hedgehog population in rural areas and a third in urban areas since the turn of the millenium. One of the reasons for their decline, as with so many endangered species, is the loss of habitat and territory. They can cover quite large areas for such tiny creatures and the introduction of domestic fencing inhibits this.
In response to the study the authors created a campaign called Hedgehog Street and in it they outlined how one of the measures that can be taken to help hedgehogs is to create a hole in your fence and add it as part of a 'Hedgehog Highway'. The study showed a difference in decline between rural and urban areas with the former showing a much greater decline. It is extremely important that as much fencing in urban areas as possible becomes as hedgehog friendly as it can.
Putting holes in fencing undermines the structural rigidity of the fence and can lead to it coming down sooner than it would have done otherwise so a little care is needed. One way to avoid this is to dig a little channel down below the bottom of the fence line and create an underpass for the hedgehog. The British Standard for how deep a fence post needs to be states for a concrete fence post up to 1.5m the post should be in the ground 600m and backfilled with concrete to half full. If a fence post has been put in to the ground to this specification digging a 150mm tunnel under your fence shouldn't under reasonable circumstances undermine the stability of the post. If it was next to a wooden post there might be a small concern about water pooling in the hole and rot setting in faster. Moving the hole slightly away should solve that though.
If digging under the fence isn't an option there are some ready made products available. Locally you can buy premade concrete gravel boards with holes for hedghogs from Suffolk Concrete and Fencing in Boxted. If you plan on visiting their yard, be warned, take some wellies!
If you have a wooden gravel board and wish to cut a hole in it make sure the fence panel is secured to the posts properly and the weight is not being held by the gravel board and you should be able to remove the gravel board to make a cut. If you wish to cut it in situ then drill a pilot hole and use a jig saw to cut out a hole. A typical wooden gravel board is 150mm wide and it is recommended to cut a hole 130mm for the hedgehog. All the load will be carried across that remaining 20mm now so I would screw a piece of batten either side sandwiching the 20mm to make it stronger if necessary.