Would buying a 1-bed now and upgrading in 4-5 years be a mistake? (London)

My partner and I are looking at buying our first property in London and would appreciate some outside perspectives.

We currently rent in an area that we really enjoy and would ideally like to stay in long term. The problem is that, at current prices, we could realistically only afford a 1-bedroom flat in the area.

We're not planning on having children immediately, but we'd like to start a family in around 5 years. That means we'd eventually need a 2- or 3-bedroom property.

Our dilemma is whether it makes sense to buy a 1-bed now, build equity for the next 4-5 years, and then sell and move to a larger place when our circumstances change. Or whether we'd be better off buying a two bedroom now in an area we are less keen on.

What I'm struggling to understand is how viable the "property ladder" really is in London these days. If we buy a 1-bed now and larger properties continue to rise in value, are we likely to end up in a better position, or could we find ourselves priced out of the jump to a 2- or 3-bed despite owning?