I think that the bandwidth distribution is restricted because periodically ,especially in the evening when TV usage is high, my unifi wifi mesh system (consisting of 6 APs) drops out and after a couple of minutes comes back on. I think the TVS are robbing the Wifi APS. This can happen 6-8 times in a couple of hours.
I think I need to do a bit of measurement to confirm.
Thanks for your help.

You're going to have to seek help in a Unifi support forum - I assume there'll be one... (since it seems to be primarily a Unifi Wifi problem),

You can't really stop your TV's using what they need. Assuming this is live-streaming (as opposed to downloading), the data has to arrive in timely manner, else it just doesn't work. It's possible that there just isn't anything left for anyone else...

The best answer is always: "don't use Wifi - cable them direct over UTP". Failing that, how about Powerline adapters? Even the cheapest, bottom-of-the-range units can probably manage 25Kbps for 4K TV. (I don't recommend buying the cheapest, bottom-of-the-range units though )