When the top dealmakers at elite U.K. law firms reminisce about the golden era which—some say—peaked around 2018, they become almost misty-eyed.

Back then, they could pretty much guarantee that the flow was so consistent that the mid-January influx would be followed up by a fresh wave of clients in the spring, hoping to conclude buy-outs before the summer break. The year had a reliable, repetitive rhythm on which they could depend.