They very likely aren't. They just appear to be to the retail crowd.

It seems to be in fact instant: https://en.wikipedia.org/wiki/Faster_Payments_Service

There is precious little explanation in the article you cite on how this is actually implemented on the back-end.

Banking plumbing is notoriously harder than most folks realize, and I wouldn't be surprised if there wasn't very hard (and very small) limits to how big of an amount / how many TPS you can actually execute with this system.

I might be wrong, but I'd be surprised if you could move 10M pounds instantaneously with this.

Stop moving the goalposts.

Got curious, for SEPA there's next day settlement and 1B EUR limit.

The instant SEPA transfers have a 100k limit.

My understanding is that this is due to instant SEPA blocking chargebacks to some extend.