Bush tucker ?
I think they might be stag beetle larvae in which case an amazing find - do you still have them?
Stag beetle larvae, deffo!!! How exciting. Hide them away again...
They look very much like Chafer grubs to me, rather than stag beetle larvae https://www.rhs.org.uk/advice/profile?pid=909
More stag than chafer. Either way no threat.
They do look very similar but given where you found them i.e. in the bottom of a well rotted tree I'd say stag beetle. Stags favour old wood stumps whereas chafers prefer living roots.
If you've got stag beetles, that is really exciting and hopefully you can put them back underground in a loggery .
(and invite me round if they come out in May!)
There's a campaign to map stag beetles and as you can see they are a relatively rare species on this part of London although more common in South West London.
@john The Romans considered the grubs a delicacy.
I saw a couple of stag beetles flying a few years ago. They were amazing - like miniature humming-birds.
Thanks very much for all the reactions and web sources. Quite separately I consulted a friend whose father worked for years in the Natural History Museum entomology department. I now hear that dad's instant reaction was "stag beetle" so we now have a majority decision! Dad also said "all stag beetles need is love and wood", "about 3 years until these grubs are beetles. If you're prepared to leave them, you can cover them back over with earth somewhere".
The ones pictured above disappeared overnight and would probably have been fox prey once I had dug over the ground. Today, I found a few more of the grubs and followed the suggestion of burying them with some rotting wood where I hope foxes won't dig them up. Only once in my life have I seen a stag beetle and it wasn't in London. This image allegedly shows the territorial distribution of stag beetles in England so we are, at least, on the beetles' home ground:
Splendid local natural history, respect to all.
I hope the 'I'm a Celebrity..." researchers don't pick this up for the next series' 'Bushtucker Trials' !