In 2017 Aviation Week published an article about M-B's two Meteors, with some interesting quotes from their chief pilot and their marketing director:-
"In spite of its considerable vintage, the sturdy British attack aircraft has all the attributes required for a stable, high-speed test platform" says Andy Gent, Martin-Baker’s head of flying and chief pilot. “From a test perspective the Meteor is ideal. The tail boom is fairly long and the fin is not very high. The engines are also spaced out a fair way out along the wing, so the efflux from the ejection test and exhaust from the gun and rocket motor isn’t potentially going down the engine intakes,” he says.
Based at Martin-Baker’s Chalgrove, England, test facility, the fleet is made up of two Meteors, WA638 and WL419, both of which have been with the company since the 1960s. “They are doing the job so why would you ever go through the heartache of getting another aircraft?” says Gent.
In short, it does not fly all too much, it gets the job done and its well built. Similar reason most older aircraft are still flying these days.
The article goes on to say that they have little intention of changing this any time soon:
Marketing Director Andrew Martin notes the company is one of only a handful that performs airborne ejection tests, and that the Meteor will continue to be used for the foreseeable future. “It is a tough thing to evaluate, and right now while we have these phenomenal assets we are not going to really think about a replacement in great detail,” he says. With the final retirement of the last Royal Air Force (RAF)-operated aircraft in the target towing role in the early 1980s, Martin-Baker acquired a large stock of spares and Rolls-Royce Derwent 8 turbojets. Because of that and the ample remaining airframe life, the company is no rush to find a successor.