In that case:-
1. The assertion that India is not aggressive is normally true. However IIRC that aggression levels are turned up a notch. Ghandi will not be as compliant as we would normally expect.
2. It is not clear that expansion will be affected. We are talking about changing the current build from warrior (which already wastes 2 shields) to a settler pre-build for a spearman. In return we get the equivalent of 10 shields an 20 food with no upkeep costs + the use of the slaves for the 30 or more turns it will take to build our next worker. Furthermore, India will be crippled, thus allowing us to expand more easily, and give us more room for expansion and allow us to defend more lightly on the Indian borders.
3. We are more prepared for war now than we will ever be. We know exactly how many units India has and we know exactly what infrastructure it has and we know that it is building a settler - or at least has just completed one. Knowledge is power let us use it.
4. India has a resourceless UU. Trying to take out India from Chivalry onwards could be a nightmare. We need both horses and iron for knights. This is C3C and strategic resource density is MUCH lower. Combine this with being hemmed in by the Indians and our chances of building knights look slim.
5. "Our primary goal is expansion". Agreed! For expansion you need room and/or military strength. We have neither. Condemming ourselves to 150 turns painted into a tundra filled corner with 1 lux and no resources is not my idea of setting out to win a game.
6. The war will be over in around 8 turns. For the sacrifice of a few turns on a settler build and maybe re-improving a pillaged tile or two we get room for expansion and a puppy dog neighbour. We get a lower defence budget because of lower need to defend, or alternately a higher offensive capability. We get faster productivity growth due to the improvements the slaves will make and in turn faster settler factory set-up.
Potentially, we get a new city, more gold, tech parity and the security of the cattle tile to the north west.
7. England are possible the biggest warmongering nation in history. England learned much from the Romans and Vikings and others later. A war now has historical resonance and should please warmongers, traditionalists, and even builders alike.
Because it "feels scary", or because "it isn't what I usually do" or because "I prefer a peaceful strategy" are not arguments against this strategy. This game is a puzzle - we solve the puzzle. The solution is not to do what you "want" it to be, but rather what is needed to provide the bottom line. I would like that to be as impressive as possible.
I move that we take this opportunity in both hands. We are English Lions after all and we should roar!