OP, I empathise entirely, we are in a similar position. Dh and I are both attorneys; we both have our own practices. I am a solo practitioner but my DH has a partner and, now, 50 employees (he has been super-successful in a very short time frame.) We have a 5 year old and a 2 year old, and I am expecting number three.
DH has very little flexibility over his hours, despite being his own boss (nominally!) as he is in court 3, 4, 5 times a day, 5 days a week. His is a volume practice. I work part-time, approximately 20-25 hours a week, and I schedule work and client meetings around my commitments with the children (baby gym, volunteering in Kindergarten, etc.) I do all the drop-offs and pick ups. During the week, DH gets home about half an hour before our 2 year old's bedtime, and then goes back up to the office after I go to bed. It's a brutal schedule but he really enjoys it, and the financial rewards. I imagine your DH will continue to have a similar workload.
One of the main reasons I am self-employed is to allow myself the flexibility to do "mama stuff" and "work stuff". I worked my tail off in law school (with a baby for some of it) and frankly, I am not willing to let that slide. In my profession, taking 2 or 3 years off is very much career suicide. To that end, I have made a conscious effort to "keep my hand in" even though sometimes I feel as though I am pulled from both ends. I accept that working 20-25 hours a week rather than 40-45 is limiting my income (hugely!!) and my ability to market my practice in the way I would like. My husband supports me entirely BUT my intention, once the children are all in school, is to crank it up to full-time. There are many times when I feel as though I need to outsource - we have a part-time nanny and a cleaner and frankly if I could get someone to make dinner for us I would! But I feel it is crucial - absolutely crucial - to not stop practicing. I also work in a very crowded field and know that my referral sources will dry up (and forget about me frankly) if I turned my back on my practice for a couple of years. So I would be very, very cautious to do anything except dial it back a little.
Having said all that, I think there are things you can do to protect yourself. DH and I earned about the same when we were courting and then married. Five years after starting the practice, DH earns a huge amount of money (high six figures), candidly, and I earn very little. I do little things to redress the balance. A portion of his monthly draw goes into a separate bank account for me, that no-one else can touch. Our finances are entirely joint, except for this one sole account that I have. Handing over financial autonomy to my DH was an enormous deal for me, and something I struggled with.
I also concur with the previous posters who advised you to consider the "worst case" scenario of DH disappearing. I spoke with a family law attorney and asked "what would I get if we divorced tomorrow?" (which gave my DH a shock when I told him I'd been to an attorney
) I am in the US and my state guarantees me one-half of all the wealth that was built/earned during our marriage. DH started his practice when we were married - although the business interests are in his name, I am entitled to 50%. I could actually force him to liquidate his share of the business if I so choose (not that I would). I would also get hefty spousal maintenance. Trust me, I kid to my DH could not afford to divorce me... Also consider a post-nup agreement if they are enforceable in the UK. I was actually minded to have a post-nup, until my consultation with the family law attorney (I don't feel like I need one, as it is.)
Good luck. This is a huge decision with very far-reaching consequences, so I understand why it is playing on your mind so much!
(and sorry for the essay - can you tell this is a hot topic of mine?)