Lots of homes and businesses have HomeHubs which share a little of their WiFi but are not listed as hotspots.
I find wherever I go I often pickup one using the BT Wifi app.
If it is commercial BTWifi hotspots you are asking about it is unlikely that you will get an answer to that question here. This is a BT residential customer to customer forum. Your post does not go to BT. The only BT staff are the forum moderators who do not necessarily read all the posts.
You could try calling BTWifi Help line 0800 022 33 22 who may be able to answer your question or point you in the right direction to get it answered.
That is a question that can have many answers. Is it out in the open, is it through brick walls, is it through glass, is it through metal framed building etc etc. As a very basic answer, less than 10 meters.
The hotspot finder shows around 100 clustered around Halewood station. They might not all be accessible but the number suggests quite a few should be.
It doesn't show individual hotspots any longer, just a pink shaded area. Probably due to the increased numbers but it's rather hit or miss.