Here I provide links to resources as there is no simple answer. What works for a professional cyclist who can keep warm through power output won't work for a casual cyclist. Generally try and buy quality once rather than buying - for example- the shower-resistant cheaper version.
The advice below will be generic so think about your own situation.
- If on a budget many of your existing clothes will suffice such as your walkers waterproof or mid-layer.
- If you have good circulation keeping your core warm will let warming blood heat your hands and feet
- If you have very poor circulation then extra warm gloves and socks will help. You might find it helps to wear long warm socks and even cycling boots to protect cold feet
- As rides often include a coffee break think how you might use that. Takes spare socks and gloves so you can start the return feeling more comfortable
The Walk, Wheel, Cycle Trust is the new name for Sustrans.
Cycling UK is my go to for advice as it fits the typical leisure cyclist. It is easy to overlook the extra care your bike needs.
I am not a fan of endless product endorsements though sometimes we need to at least know what is out there.
British Cycling; keep warm
British Cycling; warm hands
British Cycling; warm feet
Admittedly this focuses more on the committed lyra-clad enthusiast but the principles are the same.
Hope that gets you thinking.