Remember that the amount you have coming in will be subject to fluctuation on the exchange market so what 2600 is now may be more or less depending on the exchange rate.
To my mind, even if Spain is cheaper on some things, it is a wise to look at everything being like for like. We have spent the last year ( excluding mortgage and associated costs) lived off here in the UK what we would have to live off in Spain, that way we have been able to identify waste etc. We will have a very modest income, but we know we can live off of it, we have also allowed for inflation over the coming years, and not allowed for interest on savings, so basically looking at living off the least and expecting costs to rise.
If your husband is travelling to and from England and there are 5 of you 2600 seems low, but then my kids are 30+ now so what do i know LOL
Thats aside from residency, but to be honest in your situation, it seems to me that residency is the least of your issues, you need to think about schools and health first, can you afford private health cover, and schools - not sure about schools though, do you have to be paying into system to get free schools??