Hawker Siddeley Andover C Mk 1 Serial XS644 served with 51 Sqn on various SIGINT tasks for a number of years. Although it's impossible to determine the actual use of this aircraft from the photograph, it was most likely used for training intercept operators.
A Handley Page Hastings was also used by 51 Sqn for a number of years, during which it was used mainly used for a variety of equipment tests.
A Vickers Varisty Serial WJ911 was also on the strength of 51 Sqn for some years during which it was mainly used for training personnel for SIGINT duties.
In 1953, after the loss of an ELINT Dakota, Sweden purchased a Tp-82 Vickers Varsity. This aircraft was operated as a ELINT platform by the Swedish Air Force for a number of years and was frequently photographed droning slowly around the Baltic. In 1960 he Varsity was replaced by 2 second hand Aerospatiale SE210 Caravelle's. The Varsity is slowly rusting away outside at the Swedish Air Force Museum at Linkoping.
A Beech-99 was converted for ELINT by Chile.
Japan used a Kawasaki built Neptune known as EP-21J for SIGINT duties for a number of years.