So those channel numbers are “internet” channels. ie BT Sport / Entertainment / Cinema etc. 1-300(ish) is Freeview. Essentially, all of your freeview channels have disappeared.
This could either be an issue with your aerial, an issue with works ongoing with the transmitters (check freeview.co.uk) or an issue between aerial and BT TV Box.
I’d check Freeview U.K. and if no, call BT
Typically you have the aerial lead going into the Youview box & then out of that box & into your TV.
Alternatively you have a splitter that splits the input aerial cable to feed the TV & Youview box separately.
Which arrangement do you have?
And with either arrangement does your TV pick up the normal Freeview channels?
Retuning has always been straightforward until this last one. I have moved house recently, however, setting up BT was successful and I had all of the channels. The problem occurred when BT sent me a mini connector kit to overcome lack of Ethernet cable connections. I did retune directly from the Hub and not this kit. My Sony Bravia TV is connected to the aerial. An HMCL cable?connects the box and tv.
The aerial must go into the Youview box first and then you have a a separate lead from the output connection on the Youview box to the aerial input on the TV. The alternative to that is to use a splitter where you plug the main aerial lead into the inpit side of the splitter and then connect the TV & Youview box via separate leads from the 2 output sockets.
The mini connectors are for internet connection only and not part of the aerial setup.
The HDMI cable connects only the video & audio output from the Youview box to the TV, again nothing to do with the aerial setup.