It really depends on what you want. I would like to say its worth it for the money, but I see you are married and have a kid, so long term wise you family will have to move there as well so I don't think there's much 'savings' per say.

And if your plan is not to build a life there, what else is the advantage of going? Career-wise, it seems like this will be a great move. Even if it does not last, the name on your resume will be helpful and I do not believe you won't find something similar given how niche your skillset is (based on what you say).

But company name aside, is it a bigger / higher role? Because given the nature of London, you might be a big fish here but going over there, you will be a small one and unless you are darn sure you have to capability to grow to be big, it might be a waste and detrimental to your current growth in your role.

If the wife and kid is coming along, you will have to do a review of your COL with the family in tow, rather than individual.

I think its a good move career-wise, but not really sure financial wise it's much of a difference.