Here are the frequencies for much stronger signals on Sky, although they do have to be accessed via 'Other Channels'
ITV various options Frequency 12.402
Ch4, E4, TTon4, More4 etc. Frequency 12.480
Hope I'm not teaching grandmother to suck eggs but here is how to do it.
On Sky remote:-
Services
No 4 system setup
No4 add channels
frequency 12.402 (ITV)
find channels
select ones you want
save
then repeat process using frequency 12.480 for Ch4 etc