Most of London doesn't have catchments, and there's only one or two schools in Wandsworth (like Beatrix Potter in Earlsfield) and it's pretty obvious from the website. Maps are published and the road boundaries of the area described, so you will know if you are in it or not.
(Also the entrance criteria will read something like: LAC/SEN, Siblings in catchment, other catchment, other siblings, others)
Chances are, you'll just be on distance
Here is the list of greatest distances offered for Wandsworth
www.wandsworth.gov.uk/media/5813/how_places_were_allocated_for_primary_schools.pdf
there will be a similar parallel document if you're in the Lambeth bit of Streatham
Primary school age DC are entitled to free transport to school if the journey (by safe walking route) exceeds 2 miles.
The greatest distance offered varies each year, depending on how many DC are living nearby. Keep your eyes peeled at school run o'clock and see what uniform your neighbours' DC are in - that might show you which schools have offered places to your area in recent years.
And above all, don't panic! People move in and out of London all the time, and places do come up even if you don't get the offer you hoped for on Offers Day
You apply via the council of the area you live in. All London boroughs have 6 slots on the form. You can put down schools in any borough, so if you live close to the Wandsworth/Lambeth border, don't rule out schools from the other side