I don't think it's unreasonable to ask him -
a) How much will you need to earn before you feel it's enough to have children?
b) How long do you think it will take to achieve that and is that realistic?
Those questions really rely on trust and honesty and I hope you have both of those things in your relationships, otherwise you could be with someone who's stalling, who'll never be ready.
It is extremely important to be ready to be a father, but that might come as a sacrifice to you too. I was fine at 26, but in my late 20s I developed pretty bad migraines - ones that would disable me for a whole day, and sometimes leaning into the next. They progressed until I was in some sort of pain every other day, and lack of sleep is a factor in them. If I had had a baby at 26, I would be none the wiser of what awaited me. I went from being one of them most headache free people I knew (having a very mild one every 2 - 3 years or so, or even less), to full blown migraines, and my parents never had them so I had no clue I'd get them. I've had scans done so I know nothing's growing up there - it was just how my body developed. You may also get other health issues you don't presently know about.
I understand your boyfriend is being sensible, but they are things you need to consider, and accept the risk if you continue with this relationship.
How much is he earning now, and would you considering yourself to be living in an expensive area compared to the rest of the U.K?