I have a valid multiple-entry Schengen visa issued by the Spanish Consulate in Mumbai, India. I will be travelling to Spain on the 26th of March and then to the UK on 31st of March. Would I be able to travel to France when I leave the UK?
A Schengen visa obtained by any of the Schengen Area member countries allows free movement to its holder within the whole Schengen Zone regarding the European Union Schengen members as well as the EFTA Schengen members, up to its validity and timeframe.
So the countries you can travel using a schengen visa are
Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden and Switzerland. Plus Bulgaria, Croatia, Cyprus and Romania
The visa policy can be found here : https://en.wikipedia.org/wiki/Visa_policy_of_the_Schengen_Area
These countries apply the common visa policy for short stay visas
From these, Let me conclude that you can travel to Spain, France without restiction since all of them are schengen members but UK is not a schengen member.
More details can be found here : eeas.europa [pdf]