Yes, you just connect the TV aerial to IN-UHF and then TV1, TV2, TV3 and TV4 can be used to feed your TV receivers. You can ignore the IN-FM and all the stuff about VHF and FM/DAB radio.
If by any chance the BT Youview box still then feels the signal is too strong for it then you could try an attenuator on the feed to the Youview box.
If you Google for 'CPC farnell 27820HSG' you can find a page for it which has Technical Docs on the right hand side with a link to download the user operating instructions.
Thanks for the info.
I have a 6db attentuator and a variable one as well so ready in case lol.