Firstly you need good warm riding gear – having suffered from hypothermia in my youth which turned into glandular fever – not good.
The best advice it to wear multiple layers. This worked well in Utah at well below zero. I think I had 6 layers in all – 3 x T shirts. You need a neck tube or two, a balaclava works well, thermal socks over the usual ones, and a thermal midlayer with a long sleeved vest.
Gloves are a problem as very few are warm, and heated grips don’t keep the ends of your fingers warm. I used to use silk inners and thick sking gloves, which are not good to ride in, then found Gerbing heated gloves which plug into your battery. These are relatively thin but so well insulated I found I didn’t usually need to plug them in. And far better to ride in.
The temperature in the UK doesn’t usually get much below freezing although hit -17 degrees C a few years ago.
The real danger is ice, so riding at 3 degrees and below is very risky, unless it is dry. You cannot control a bike on ice, and unless you can keep travelling at constant speed in a straight line you are going to crash. Even shutting the throttle on my KTM 950 SM instantly locks the back wheel, although it might not on some other bikes. All I can do is ease off the throttle and hope to get across the ice to tarmac.
You can’t see black ice, although with experience you get to know where it is likely to occur – exposed road surface typically on bridges or in heavy shadow. But you can never be certain that you have the basic grip you need.