If you have a problem with the HH5 you should consider buying a third party VDSL Modem/router. They start at about £30/£40. As well as performing better than the Homehubs it will have the advantage that it does not transmit the BTWifi signal.
If you don't want to do that, you will need to report a fault to BT. They, like all ISPs do not guarantee wireless connections because of all the variables that can affect speeds so you need to just report it as a faulty hub, not the wireless connection dropping out.
Have you tried a factory reset of the Homehub by pressing a pin into the recess button on the rear for about 20 seconds. This can often sort out problems.
You should be able to Opt out of BTWifi using your BTID.
Some people have found that to do this though the BTID has to be an email address.
Check your status first to see if your BTID is accepted.
https://www.bt.com/wifi/secure/index.do?s_cid=con_FURL_btfon