Don't go down this road with this fear...
N. Fowleri is found in warm, stagnant water. (i.e. Lakes, ponds, wells, etc.) and you said you're in England, it's rare there. It's just not the climate N. Fowleri needs to thrive.
Rainwater is filtered in the atmosphere by the way.
You have nothing to worry about as far as infection from what you've described.
I won't even go into telling you how extremely rare infection by N. Fowleri really is, because if you know about this amoeba that means you googled. You shouldn't even know this exists!