Why does Martin-Baker use a Gloster Meteor as a testbed for ejection seats?
Aviation week did a bit on the plane last year 


  "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.




Also a bit of a side note, this is not their only testbed. MB also uses a high speed sled for testing at their Langford Lodge, Ireland facility. This allows ground testing of the seats at flight speeds. They also test static scenarios in what appears to be cockpit mock ups as generally speaking seats are certified for Zero-Zero use. Varying governing bodies may require different testing and live demonstrations as per their certification process leading to different testing procedures.