Using 'open' access Wifi point instead of connected private wifi.

I was at a home today and connected to their private home network on a Surface Pro 4. This showed their network as Connected. However, the internet kept going to a local open BT network instead! I couldn't select this open network anywhere to exclude it. Thinking about it now, I should have disabled/reenabled Wifi first, and then tried a reboot after that. But otherwise, is there any hidden setting to ignore certain networks?

Hello and welcome to the TenForums :)

If the home where you were had a BT hub then it most probably was offering both an open connection also known as BT Wifi or BT Fon and also a secured wifi connection which would have an SSID number with a BThub name with some random numbers and letters. It is possible that you connected to the open one and were then automatically directed to a BT broadband page on the internet to pay for use of the BT hotspot service that they provide via customers routers. See this article for more info: http://bt.custhelp.com/app/answers/d...fi-with-fon%3F

Yes, the SP4 connected to a BT Fon connection somewhere. However, the Wifi settings said I was connecting to the home's private Wifi connection, for which my friend put in the password.

So, Wifi was showing available:

Cheapside_WLAN
BT_FON
Some_network
Some_router
Several_others

I selected Cheapside_WLAN and put in the password. This was shown as connected. I tried an internet page and got the BT_FON login page. I tried a couple of times including opening in a new tab and a Google search and a couple of times, Cheapside_WLAN was used. But then it stuck with BT_FON no matter what I tried, trying to download a file.

At this point I should be able to exclude a network, surely? But though there are things like 'forget network' for known details, I couldn't find 'never use this network' anyway.

In the W10 Manage WiFi settings there is a section for Paid Wifi services and you can turn this off. It will prevent the device from temporarily connecting to open WiFi hotspots

It's already off.

Okay, I think it's a bug. It appears Windows is using this public Wifi setting despite it being disabled. I've had similar false positives with Airplane mode being shown active when not.

When I connected to the home Wifi, a notification asked if I wanted to use this network. I'm guessing now that that was a request to use the public hotspot, that just happened to pop up after I put in the password for Cheapside_WLAN, so I probably inadvertantly accepted use of the access point. Although why that was prioritised over my intended connection, I've no idea.

If it happens next time I'm there, I'll try the old switch-off-and-on-again solution.

Sounds like you are making progress on this. Wifi connections can be affected by interference and that is why there are different channels to choose, sometimes the channel selection is on automatic so when you first set up a connection it may be searching for the best channel and take a little while to settle down. Usually once everything is working it should be stable.