I will travel to London (LGW) this Sunday and I'll take public transport to Walthamstow Central. I would then like to take a taxi to my final destination.
Is it likely to find a cab at aproximately 00:30 or is it advised to call one?
There are a number of taxi (black cab) ranks around Walthamstow Central Station (scroll to the bottom of the link):
Taxi Rank Selbourne Road (North side of Walthamstow Central Station) Waltham Forest
Taxi Rank Selbourne Road (East side of Walthamstow Central Station) Waltham Forest
Taxi Rank Selbourne Road (Walthamstow Central Station) Waltham Forest
Usually there are a couple black cabs lurking around at all times as it is a large interchange hub composing a tube terminus and a bus station.
However from a price perspective it is almost always better to book a minicab (officially known as Private Hire Vehicles) as they are generally cheaper over the same distance - make sure the minicab company is TfL licenced. The "ride-sharing" company that starts with a U operates as a minicab company in London, for example.
N.B.: Minicabs are cabs that can only be booked, but not hailed on the street; although "booking" is often as simple as walking into a minicab office and asking for one (but doing this at busy times means you'll probably be waiting for a while).