Imo people are being a bit unrealistic telling the op to move to a smaller house - in the current climate moving isn't exactly a pheasible option for most people considering houses aren't selling.
Also I'm not sure that comments of "well think yourself lucky" and "well we lived on far less than that" are really that constructive. Yes the op is financially in a much more comfortable position than most people, but fact is that people do live to their means, and if you have a comfortable income it is much easier to spend more because if you know the money is there then you spend it without worrying iyswim, and before you know it you're spending to your means and beyond.
Op, if your dh is earning £100k a year then he will be bringing home approx £4.5k a month? In most situations you should be able to live on that, and if you can't then you seriously need to question why, and look at what is going to give in order that you are not left struggling every month.
Firstly, How big is your mortgage?
Realistically your mortgage isn't something you are going to be in a position to change for a while due to the current economic climate and the position of the housing market. Given you have a large mortgage I'm assuming that you don't have much equity, and that is something that is going to prevent you from selling your house for a while IMO. So first and foremost take your mortgage off your monthly income, and then we'll look to change the rest.
Next outgoing is council tax. Again this isn't something you're in a position to change. Ditto with water. Although if you're currently paying rates, then installing a meter is a way you can save as you pay for the water you actually use rather than a flat rate (and this is marginally cheaper.
Next you need to look at your energy bills. Firstly, are you sure you're with the cheapest supplier you can be with? Go and have a look at the suppliers, and see whether there is anyone cheaper than your current one. It really is possible to save hundreds of pounds a year.
So those are the bills you don't have all that much control over, so the rest of what is left is what you spend on other things, groceries, cars, incidentals.
Firstly cars, why do you need two cars? If you don't work, do you absolutely need a car? do you live in the country with no public transport? Because imo that is the only reason for absolutely needing a car. How far from home does your dh work? Could you drive him there and use his car and then collect him from work/encourage him to catch a buss home? I would look very seriously into getting rid of one of your cars, because you will almost certainly save a couple of hundred pounds by doing so.
Next look at groceries. £100 a week is a lot of money to spend on groceries. What are you spending it on? Again it's an easy trap to fall into. You go into the supermarket and see special offers for the products you might not have gone there to buy, and before you know it you're spending more money than you had intended. Here's what I would do:
I would do one monthly shop. for all the bulk items like the nappies, the big bags of pasta, the breakfast cereals etc. I would buy own brand products where possible, generally they don't taste that much different, but in terms of price you can save a lot. Did you know for instance, that a jar of own brand chocolate spread is almost £1 cheaper than nutela? And it really doesn't taste any different (the kids certainly don't know the difference). I would also aim to shop online so you're not tempted by the offers you wouldn't ordinarily buyh. So, given you'll have a freezer and cupboards full of food, I would then draw between £20 and £30 a week in cash out, and use that to buy any other shopping, bread/milk/fruit/veg. It will give you a far greater understanding of what you're actually spending, rather than just putting your card in the slot knowing the money is there iyswim. And aim to havemoney left at the end of the week, and if there is, take it off next week's cash withdrawal, so if you draw out £30 and you're left with £1.50, next week you get £28.50.
Right, now you've looked at all that you need to look at the things you have that you don't really need or that you can cut back on.
Firstly, do you have sky? If so do you have full package? and if so do you actually watch it? If not I would cut down to minimum possible, and that will save you probably about £15/20 a month.
Next do you have broadband? If so with whom and what speed? Again, you san shop around for broadband. There are lots of deals, and I would opt for the lowest bitrate because that will cost you the least amount of money. It's unlikely you're going to be streaming video across your broadband connection, so lower speed is perfectly adequate.
What about other items? Do you have mobile phones? Contract or pay as you go? If contract do you actually use enough calls/texts to justify your £20/30 a month on your contract? If not switch to PAYG and top up by the minimum possible...
Once you've done all that you really should have money left, even if it's a couple of hundred pounds to buy clothes/save towards a holiday.
I do know how easy it is to fall into that trap, and to become complacent about money, but on that salary you should be able to survive and have money left at the end of it.