The other issue for SIM-less 999 calls is that the operator can't get a number (as it doesn't have one) and so they have no way to call back if needed. As said, the phone may well think it can make an emergency call, but it doesn't mean it gets connected.
In the US, network operators are legally obliged to connect SIM-less phones to 911 - not so here.
As for the call itself, ALL UK emergency calls go operators in one of five centres - they're the ones who ask "which service?". For landlines, they get the full street address and for mobiles a rough location based on cells it's near (with an indication of accuracy - at worst within half a mile) - so they can route the call to the appropriate police / fire / ambulance / coastguard / etc centre nearest the caller.
This is all the same whether you use 999 or 112. I've come across misinformed people running first aid courses too - there must be someone training the trainers who's perpetuating this myth.