Check your router setup, make sure this check box is clear (No Tick) Advanced > Setup > WAN Setup > Disable IGMP Proxying
Freeview are frequently adding new channels, re-structuring and moving things around. It might be worth trying a re-tune of your box if channels are missing. Your box gives a good indication of signal quality of the channel you're watching in Settings > TV signal quality. Note that signal quality is more important than strength
There is some help with signal levels on this page Picture breakup or missing FreeView channels
You will need to connected to BT Broadband to access this link, and any other links I post.
Sorry to hear your sports channels are not working for you.
If you PM me your telephone number or account details I can take a look into the issue for you.
You should have a lot more than 18 channels over aerial, you should have the same number as if you were watching with the aerial plugged straight into the TV. Have you tried re-tuning the Youview box?
Probably meant to be the other way around, typo?