Stag Beetles are Britain's largest insect growing up to 7cm or 4" long.
Although scary in appearance they are a timid creature. The females are harmless, the males might give a light nip with the pincers if panicked but I've handled several and never come to pain.
The larvae benefit the ecology by eating rotten wood. The adults do not feed much and are vegetarian.
Stag Beetles are an endangered species though they do better in London than in other parts of the country.
They tend to appear in May or June, sometimes as late as August, usually in the evening.
Females can fly but rarely do so, males do fly and are a spectacular site.
If you think you have beetles in your garden, then take care where you step in the summer months. Although black/brown they are often look like a bit of garden debris and are easily trodden on. Also do a scan of your lawn before mowing it.
If you find a beetle which is in harm's way then you can gently pick it up. Males might nip but this is unlikely and if you pick up your beetle by the carapace/shell then it can't flip round and get you. Put it down somewhere safe but not far away.
Stag beetles bumble around especially in flight. If you go out for a close look then make sure they don't bump into you. If they do they'll stop flying and settle on you! Don't panic, lift the beetle off gently making sure it's feet aren't stuck in the fabric of your shirt/blouse/top etc. and put gently down.