Any question answered...

Clearly the council know the sign has been removed and the hole repaired. I would say that the council are liable as the absence of a bollard has caused the incident just as they are liable for a pothole with council white paint around it. Once they know about a defect they have a duty of care as far as I can see.
Have a look at https://www.blclaims.co.uk/latest-news/news/2013/01/18/highways_and_potholes/
also search google for - Highway Risks and Liability Claims

Take photos of any other islands in the same street with the bollard/s in place to prove your point when required.

Turns out there was originally a neon yellow plastic bollard thing on the island, which was knocked off and the hole just filled, with it not being replaced. The other islands along the road do still have them, so more promising that Joel can get someone to pay for his teeth.

That's unusual and (as Constable Savage says) could strengthen your case, as it points to problems having occurred there before. I'm not sure what sort of bollard you mean, but I assume it wasn't an illuminated bollard, as they're only really meant to go on central islands (so that drivers expect to drive to the left of them, which doesn't prevent them from crashing into them quite often). If it was some other type of bollard, that was clearly a bodge and not a very intelligent one--as you say, it was crashed into, which shows that it was an inadequate response to the problems there. Build-outs like this certainly shouldn't have bollards. They're not big and conspicuous enough to help and can cause an additional injury risk, even if they're plastic. You can put big obvious things on large build-outs--recycling bins or even trees--, things that aren't hard to spot, if the build-out is big enough, and this one is obviously too small. As it so often happens, there may not have been money/urgency to do a new scheme there soon enough.

You could also explore why the car parking seems to be empty so often (judging from your incident and the suspicion above as to the reason for the bollard). Perhaps it's mainly used for the adjacent playing fields or something else out of shot only in the daytime and not at night?

You can get historic Street View images of some places which might help your case. I used this for a pothole claim.

Yup yup yup, seems like they're supposed to be there as seen here further down the road.

Edit: as noted by many others prior to my post...