Want to buy a bigger house in 10 years. What to do with money in the meantime?

Hi guys! Bit of background...

The wife and I (both 31) live in a 3 bed house in an average location where we have paid off the mortgage. It’s worth about 240k.

We have one new born baby (wife on maternity leave) and we will possibly be looking to have another in a couple of years.

When they are both a bit older we would like to move somewhere a bit bigger and a bit more rural. Looking at prices this would probably cost us £450-500k plus stamp duty etc.

Between us we earn (when wife not on maternity!) about 70k.

Ideally we would like to buy our next house with as little mortgage as possible (sub 150k).

I guess my question is..what would you do in this scenario and should I be staying clear of index funds as I need the money in 10 ish years? Currently our savings are not too much so are covered by high interest savings accounts.