Great video and information. I live in suburban south east London and stag beetles breed in my garden, in an old pear tree stump and a tree of heaven (Ailianthus) stump that has honey fungus in it. In the UK most Lucanus are found in Gardens and Parks, very rarely in woodland. They don't grow as big as in Germany or Spain, here average males are around 5cm length. I have found 7cm males in the past but the larger specimens seem to be rarer now. I did find a 4.5cm long female L.cervus on the pavement near my home last week, that's about as big as females stag beetles get in the UK (in Germany and Spain to 5cm in females).
The biggest threats to Stag beetle populations in the UK is loss of habitat as people concrete over their gardens for car parking or patios. People also have tree stumps removed rather than leaving them as in the past. We also have a much larger Magpie population now as their numbers are no longer controlled and they wait around for the stag beetles to emerge from the ground and eat them.
A few years ago my son Patrick found a male stag beetle dead in the road, it had it's aedeagus extended so I put it in my collection. I have an 80mm male from Hungary and I know that exceptional specimens grow to around 90mm. In the UK, specimens over 70mm are rare.
Great filming of you with the stag beetles, I visited your website too and was interested to see your breeding program. Lucanus has always been one of my favourite groups. Do you collect many other insect groups?