A properly engineered/configured fixed wireless network will have excellent latency. Typically 1 millisecond per hop.
The total latency will of course depend on the quality of the backhaul (connection to the internet itself) , i.e. Connected to a Fibre PoP (point of presence) or well connected Data Centre.
12km is not a problem distance at all, especially for clear line of sight.
I have no experience of , or background on AB Internet themselves so your experience may vary. however escaping the BT line is something worthwhile exploring , you may never go back!