Agree your first port of call is a solicitor. Look on the Law Society Website and choose find a solicitor. Put in family law and your area and a list will come up.
Do choose carefully and choose someone who specialises in divorce and who is experienced at dealing with high net worth individuals - their biography should say so. Family law is a broad church so read carefully.
This specialist knowledge is more important than them being near to you. You only need to visit them once, the rest will be done by email and telephone.
Ideally don't choose one based in London because the fees are higher without necessarily better service. Personal experience here - I changed lawyers part way through my divorce because I was dissatisfied with my first quite well known divorce lawyers). I can recommend a good out of town lawyer if you are in the South East.
Your solicitor should have a darn good idea of what you will receive based on precedent. If they are in any doubt they should refer you to a financial adviser specialising in divorce settlements. Again a divorce lawyer specialising in high net worth individuals will know one of these.
Do not listen to the advice you get on here about spousal maintenance which is invariably wrong for HNW. Any deal should give you either additional capital and/or spousal maintenance, do not accept just child maintenance. Remember you are entitled to a share of his pension too. Agree that housing the DC plus you as the main carer should be a priority and that may well mean you staying in the family home - houses are not selling well at the moment and you need to make that point rather than end up with a settlement which assumes an impossible house sale.
It sounds as if your H is planning on being a dick so make sure you do not take his word for anything. Do NOT try to be nice and keep things amicable by giving up or reducing your claim, all you will do is shortchange yourself and your DC.
Be prepared for him to suddenly claim a burning desire to have his children 50:50 so that he can reduce or avoid paying child maintenance.
Before you meet the solicitor you will help yourself by getting together as many details and copy documents as possible of your assets and drawing up a rough list together with approximate values.
I can give some input on how to manage legal costs going forward at some point if you like.