Suppose that sometime before 1934 Pyotr Kapitsa http://en.wikipedia.org/wiki/Pyotr_Leonidovich_Kapitsa http://www.britannica.com/biography/Pyotr-Kapitsa decides not to make any more return visits to the USSR, rightly fearing that he would not be allowed to leave. Effects on both Soviet and British physics? (When it comes to Kapitsa's importance to Soviet physics, there is not only his own scientific work and his direction of the Institute of Physical Problems, but also his role in helping to save the lives of Vladimir Fock https://en.wikipedia.org/wiki/Vladimir_Fock and Lev Landau, https://en.wikipedia.org/wiki/Lev_Landau both of whom had been arrested during the 1937-8 terror.)
In OTL, Kapitsa refused to work on the Soviet atomic bomb program (for which he was put under house arrest for several years). He later explained that he was not a pacifist and did not object to the bomb on principle--he just objected to working under Beria. So presumably if Kapitsa stays in Britain, he would not necessarily refuse to help build a British atomic bomb...
In OTL, Kapitsa refused to work on the Soviet atomic bomb program (for which he was put under house arrest for several years). He later explained that he was not a pacifist and did not object to the bomb on principle--he just objected to working under Beria. So presumably if Kapitsa stays in Britain, he would not necessarily refuse to help build a British atomic bomb...