From the book Viscount, Comet & Concorde by Stewart Wilson -
"With the slowdown of the Viscount development programme over the northern winter of 1947-48 came the decision to complete the second V.630 prototype as a testbed powered by two 6,250lb thrust Rolls Royce Tay centrifugal flow turbojets in place of the usual four Dart turboprops. Redesignated the Type 663, the aircraft flew for the first time from Vickers' Wisley airfield on 15 March 1950 carrying the military serial number VX217. The allocated civil registration G-AHRG was not taken up.
The 663 retained the same overall dimensions as the V.630 Viscount prototype, the changes incorporated revolving around the installation of the two Tays which were mounted in underwing pods each incorporating a revised main undercarriage design. The Tay-Viscount made only one public appearance, at the 1950 SBAC show in Farnborough where it caused something of a sensation with the turn of speed and rate of climb it displayed. Performance figures were never published.
The aircraft was used by the Ministry of Supply for various tests and trials including high altitude tests of the Boulton Paul powered control system for the Vickers Valiant bomber. It then became the first aircraft in the world to fly with an electrically signalled flight control system (fly-by-wire these days) and was also used by the Decca Navigator company.
The Tay-Viscount survived until 1958 when it was damaged beyond repair in a hydraulic fire having logged just 110 very useful flying hours."
Hope this helps!