Well, I live in the same area and I didn't have to revise for GCSEs because they're easy to get As in anyway, or were back then, but I had to work very hard for my A levels and had no space at home, too much noise, baby, adults fighting, tv blasting, etc. — just like many boys at that school (which has a high intake of poorer children from the very deprived local area) — so I simply sat in quiet spots on Hampstead Heath to revise.
Really not difficult and doesn't require £20,000. Perhaps £50 - £100 for cafe snacks if it's a rainy summer.