You pay what you need. If one page doesn't need $latex$, it will not load MathJax.
Add that article like
+++
title = "use latex"
date = "2022-01-16"
[extra]
latex = true
+++
$$1×£1 + 1×50p + 2×20p + 1×5p + 1×2p + 3×1p$$
$$1×£1 + 1×50p + 2×20p + 1×5p + 1×2p + 3×1p$$
inline latex $latex$
inline latex $latex$