That timeframe for dropping would suggest to me that it's struggling to cope with demand, as that's when most people at home and online. That leans it toward 'problem with BT's network' for me, but you'll probably have to jump through the hoops before they'll accept that, so try it anyway.
I would also, if your devices are capable of it, connect them preferentially to the 5Ghz band, as it's generally less prone to interference and more stable. That won't help if it's all just overloaded but if the problem is that, for example, all your neighbours are switching on their routers when they get home from work and the additional wireless networks interfere, 5Ghz should be more stable. (You shouldn't ever turn your router off, btw, if you can help it. Your provider sees that as a drop of service on their end and assumes the connection is unstable, then reduces the speed in an attempt to stabilise it - which is unnecessary if it's not unstable, just switched off). Also, thinking outside the box, is there anything near your hub that you only use in those times? Cordless phone, microwave, baby monitor? All those can interfere, move the hub away from them.