The basic premise of this scenario is that at the end of WW2 British rather than American troops succeeded in capturing data and prototypes from the Gotha aircraft factories at Warnemunde relating to the Horten Go.229. They also captured data and engines from the BMW jet engine factory in Brandenburg, including an intact 018 prototype engine, along with a number of Jumo 004 engines from Junkers.
The engine data and hardware ended up at Rolls Royce, where Stanley Hooker's team wasted no time in applying their more advanced metallurgy to the designs to produce improved versions of the 018 and 004 ( the BMW 003 was dropped as RR already had a similar thrust engine in the Welland ).
At the same time Armstrong Whitworth became custodians of the Go.229 prototype and other flying wing data, their interest stemming from their existing AW.52 flying wing project.
When in late 1945 it became apparent that the existing PR.XIX Spitfires would not be able to penetrate unfriendly airspace for much longer without being intercepted by jet powered fighters, the Air Staff issued a requirement for a replacement which "should be capable of reaching sufficient speed and altitude to avoid interception". AW submitted a proposal based on a lightly modified Go.229 powered by RR produced 004 engines ( with vastly improved reliability over the original versions, plus slightly increased thrust at 9.1kN ), and stated that as it was based on an existing airframe and engine combination development time would be minimal. This short development timescale was publicly acknowledged to be the deciding factor, though later information indicates that the low radar cross section of the airframe was also a major contributor to the decision, though quite when and how this aspect of the design had been discovered is still unknown.
Modifications to the airframe were mainly in the area of the undercarriage, with the original gear being replaced with something more compact, though this restricted operations to paved runways, and the twin 30mm cannon were also removed to reduce weight. These modifications freed up space in the aircraft for more fuel as well as the recon camera installation. The added fuel increased the range to around 1000 miles radius, with most of a typical mission taking place at over 50,000ft altitude.