Hello. You would at least need to be based in a country where WaterAid has an office, for tax reasons. For the actual city, I'm not sure and you'll need to ask those actually doing the recruitment!
All the best

London-based position will be initially home-based due to restrictions on office numbers.

Could you expand a bit on the "home-based" aspect? From reading the job description it does not seem to be necessary to relocate to a WaterAid office location at all.

There's a great job advert in WaterAid, in my team, for 18 months, ideally for people with very good experience in sanitation (especially rural): more details on jobs.wateraid.org/en-GB/job/senior-advis.../J3S2FX6LXKTWC0962Z5

It says "London",  but can be in any country where we have an office.