If it's Mayfair and M&A, very likely it's PE work. I work with those guys and can confirm that the culture is crazy macho - extremely long hours, and yes usually a summer slowdown in European M&A (because most of continental Europe disappears for August so you can't take a deal to market). However, this year, the market got off to a late start (because of the March Brexit 'deadline') and so it is much busier than it usually would be at this point in the summer, with some deals still being started/prepped for market.
There are also a LOT of guys in this culture who effectively outsource family life - HOWEVER, they are the ones who can "throw money at the problem", as one colleague charmingly explained to me he planned to do when I was asking whether he was excited about the imminent birth of his first child. I have colleagues who have full time housekeepers, nannies AND au pairs, although their wife doesn't work. In addition they tend to be obsessed with gyms and fitness - client events often revolve around cycling/triathlons and the last couple of years has seen a huge upsurge in healthy "meal services" where all their food gets delivered each day to the office/home so that their whole life is outsourced.
OP sounds like she has the worst of both worlds - a DH who wants to be part of that culture but isn't prepared to (or can't afford to) pay for the home end of it.
In terms of family relationships, they vary. Most with successful marriages have a wife who knew exactly what they were getting up to, does not expect much (if any) joint social life, and is fine with having their expensive holidays interrupted by lengthy conference calls. There are a few men who make massive efforts to work from home in the evening, go to school events and even drop off in the morning. The ability to do that depends on which team you work in and how senior/valued you are. If you are a junior or mid-level without any particular brilliance or powerful contacts, it can be hard to manage that flexibility but some do.
I think it's a pretty repugnant culture and would hate to live that way myself, but the material benefits are considerable and that can work for SOME people IF the wife (and it's always a wife - I NEVER see this situation the other way around) is making sure that she gets a fair bargain in terms of paid help at home.