Como indicado @Relaxed, a Bulgária e a Sérvia não fazem parte do espaço Schengen (ainda). Eles ambos deixam você em se você tiver um visto Schengen, mas não é necessário.
Você precisa de um visto local para a Bulgária, parece que não se importa com o seu status no Reino Unido, mas a Sérvia vai deixar você entrar.
Bulgária ( referência )
Last update: 03.06.2015
Country Visa is required India YesOn 25 January 2012 the Council of Ministers of the Republic of Bulgaria adopted a decision according to which by the date of Bulgaria’s accession to the Schengen area, our country will unilaterally apply a visa-free system for holders of valid Schengen visas. They will have the right to enter and reside in the Republic of Bulgaria for a period of no more than three months in any six-month period from the date of the first entry, without needing to have a Bulgarian short-stay visa. The decision entered into force on 31.01.2012.
Sérvia
At its meeting of 30 October 2014, the Government of the Republic of Serbia adopted the Decision on visa free entry to the Republic of Serbia for holders of foreign passports having a valid Schengen, UK and other Member States' visa, or visa of the United States of America, and for holders of foreign passports having residence permit in the countries of the Schengen area, EU or the United States of America, which was published in the "Official Gazette of RS", No. 119 of 31 October 2014 and which will enter into force on 8 November 2014