What was comp like (Base + Bonus + Deferred) during the peak of the British Empire for M&A?

Corporate mergers just weren't really a thing until the late 19th century so you don't really have M&A advisory historically. That being said there certainly were creative ways to raise capital before corporations were even a thing. In feudal societies Lords who wanted to go on crusade would finance their journey by pledging the cash flows* from their lands to a lender (usually a monastery). 

*Rents would usually be in-kind, not in actual currency or gold. 

I think back then you could just finance a corporate army sponsored by the crown which would then allow you to facilitate free "hostile takeovers" wherever you'd want.

Well Creditanstalt was Rothschild's owned and backed, but aside from that... M&A advisory only really came into it's own in the 20th century when the likes of Andre Meyer was running Lazard in NY. There was a form of advisory work done before than that but we intimately linked to bond and equity underwriting which were the revenue drivers. The majority of money-making during the "golden age" of Imperial finances were pretty basic stuff today: remittance management, gold and currency transactions, and bond underwriting/lending. I do not know how much the comp would be but the private banking houses were partnerships and so the partners would expect a % of profits, although that did come with unlimited liability.

Andrew Carnegie had a side hustle for decades as a bond broker collecting FAT placement fees/commissions. Certainly paid to be an intermediary/market maker with the spreads that existed back then