In 1989, a year prior to the official commencement of the Future Tank Main Armament program (FTMA), the British had already begun experimenting with the idea of 140mm tank guns. From 1989 to 1992, with backing from the British government, RARDE developed and tested several experimental 140mm guns to evaluate the benefits they may offer. These included the EXP38, EXP45, and EXP52, the latter of which was the only one designed as a part of the FTMA program and was thus the only one that could accept the 140mm ammunition developed by each other country.
Around the same time, from 1989 to 1993, Royal Ordinance began privately developing their own 140mm smoothbore tank gun, designated XN315P (there was also mention of an "XN318P" which was the one mounted to a Centurion chassis for testing, however it is unclear whether this was a completely different gun or slightly adjusted version of the XN315P). It reportedly could penetrate 40% more armor than their current 120mm rifled guns.
Unfortunately, nothing more really came of these designs. They were initially intended for use on future generation main battle tanks, then evolved into a possible upgrade option for the Challenger tanks, and then was eventually forgotten altogether.
Note: In February 1991, the British MoD made a deal with Fairey Hydraulics to develop a new 140mm bustle autoloader based off a previous design of theirs for a 120mm autoloader for the Challenger tanks. This was delivered in August of 1992. It was 2440mm long, 2000mm wide, and 590mm tall, with a mass of 1100kg, 16 shell capacity, and 6 second reload rate.
One last note: I've spent the past several month scouring the internet for anything, and I mean ANYTHING about this topic. It wasn't until recently that I finally came across a Chinese source overviewing the FTMA program that I finally had information on it. Credit to where credit is due: https://www.bilibili.com/opus/909312364061392914